Use images only when they provide useful visual explanations of information that is otherwise difficult to express with words, or for screenshots of UIs that are important to the discussion.
Some best practices for images.
Creating and saving images
- To create a diagram, use any drawing tool.
- If you have an SVG file, use it as your image. If you don't have an SVG file, then save your image as a PNG file unless you have a good reason to use a different format.
- Make your screenshots of windows look like real windows, usually windows from a recent version of macOS unless there's a good reason to use a different operating system. For example, include the window's title bar in the screenshot. And if a given window has a drop shadow, then include the drop shadow in the screenshot as well.
Text associated with images
Some best practices for text with images:
<figure id="emu-foot"> <img src="/assets/images/emu-foot.svg" width="120" alt="An emu foot has three toes."> <figcaption><b>Figure 1.</b> Diagram of an emu's foot</figcaption> </figure>
- For a figure caption, use the form "<b>Figure number.</b> Description".
- Don't end a figure caption with a period unless the caption ends with a
full sentence. Generally try to avoid putting full sentences in captions; full
sentences are more appropriate for
alttext, and for introductory sentences.
- To refer to a figure, mention it by number ("figure 1"), to avoid spatial descriptions (that is, don't say "the image below").
- As shown above, use an
altattribute to provide a text alternative for the image, which is used by assistive technologies, such as screen readers, and might appear if the image cannot load.
However, if the image is decorative (not informative) or it's provided only as a visual aid for information that is already expressed in text, then provide empty alternative text (
alt="") so it will be ignored by assistive technologies. (If you exclude the
altattribute completely, then screen readers might instead read the filename aloud.)
As per the HTML specification, "the most general rule to consider when writing alternative text is the following: the intent is that replacing every image with the text of its
altattribute not change the meaning of the page." So if the alternative text is redundant with surrounding text or it's not useful to visually impaired readers, use the empty tag.
- In most cases, avoid embedding explanatory text in screenshot graphics; text that's incorporated into a graphic hurts accessibility and increases localization costs. If your text is important enough to be worth embedding, then you can include explanatory text in the image, but in that case, be sure to also provide the same information in a form that people with visual disabilities can use.
Modern browsers can use high-resolution images if they are available; this makes the images look better on high-resolution displays.
To provide a high-resolution image, use the
srcset attribute in addition to the standard
src attribute. The
srcset attribute lets you specify
different image assets for different screen resolutions. It accepts a
comma-delimited set of image URLs, with the target screen resolution specified
by a size qualifier:
1x meaning the "standard" resolution,
2x meaning "double" the resolution, and so on.
If a web browser supports the
srcset attribute, it selects an
image from the specified images that's an appropriate resolution for the current
display. If the browser doesn't support the
srcset attribute, it
uses the image in the
src attribute. Consequently, you must always
still include the
For example, to provide both a standard resolution image and a
double-resolution image, add a
srcset attribute and specify both
2x image assets:
<img src="/assets/images/skateboard.png" srcset="/assets/images/skateboard.png 1x, /assets/images/skateboard_2x.png 2x" width="375" alt="" />
widthattribute matches the CSS pixel size used for the page dimensions. (The height is automatically calculated based on the width and the image's proportions; don't state it explicitly.)
- Set the
srcattribute to point to the standard-resolution (
1x) image, not the
2xversion. (Almost everyone who has a high-resolution screen also has a modern browser that can recognize the
srcattribute is mainly used by older browsers on low-resolution devices, which should download the smaller, low-resolution image.) Even if your original image is the higher-resolution image, set the
srcattribute to use the standard-resolution version; don't force a reader using a low-resolution screen to download a graphic that's higher-resolution than they can view.
- The filename for the double-resolution image (in this case,
skateboard_2x.png) can be anything—it's the "
2x" value following the filename that informs the browser which resolution the file is. But it's a good idea to use a filename of the form
basename_2x.extensionto make clear to human readers that it's a double-resolution version of
- The double-resolution image must be exactly twice the width and height of the standard image, give or take a pixel. (For example, it's okay for the double-resolution image to be 875x500 and the standard size to be 438x250.)
- Don't scale up an existing
1ximage to make the
2xversion. If all you have is the
1xversion, then use it alone. But if you're starting with a high-resolution image (at
2xresolution or better), then you can scale it down to appropriate dimensions for
- Currently, only an additional
2ximage is necessary, but someday screen PPI may increase further. So the
srcsetattribute supports further alternative sizes, each specified by the appropriate multiplier, such as
- A browser that supports the
srcsetattribute uses only the images provided in that attribute—it ignores the
srcattribute. So specify all available image resolutions in the
For more information, see the HTML specification for the
Laying out images on a page
- Don't try to place an image manually; for example, don't use a
styleattribute or other workarounds to control the image's left/right justification or the margins around the image.
- Don't make your image too small. It's fine for an image to take up the full width of a page.
- Consider how the image will look when printed out.
- In general, don't use an image that's wider than the column it appears in. On developers.android.com, for example, the main-body column is 856px wide, so use images that are no wider than that. In that context, the high-resolution 2x version of the image should be no wider than 1712px.
- Screenshots at full resolution often take up too much space on the page, so you may have to resize them.
- If the graphics were created by someone else (for example, a designer on the team you're supporting), it may be fairly trivial for them to provide you with images at the appropriate size. If the images they provide are wider than 856px, ask the designer if they can provide the relevant graphics as 856px/1712px pairs.
- Don't link to the figure from within the same page unless it's a very long page and you're linking to it from quite far away on the page.
- Don't center the image on the page.
- Don't put the
To place the image inline (align left):
<figure id="descriptive-id" class="attempt-left"> <img src="" alt="" width="XXX" /> <figcaption> <b>Figure 1.</b> Description of figure </figcaption> </figure>
To align right:
<figure id="descriptive-id" class="attempt-right"> <img src="" alt="" width="XXX" /> <figcaption> <b>Figure 2.</b> Description of figure </figcaption> </figure>