
On the Web it is preferable to place your
caption in the HTML. If that won't work and
your captions are long, you should also link
to a place providing a description of the image
and an alternative rendering of the text.
Colleagues of mine are involved in a project that involves adding captions to photographs. Like many of you, they aren't full-time designers and haven't spent a great deal of time using Photoshop. While they know how to crop and resize photos, they've not yet worked with type. For those of you who may someday face the same situation, here is a quick tutorial on adding text to images.
Are your captioned images going to be used on the Web, on hand-outs produced by your office printer or in commercial print work such as a magazine? At what size will they be used? When editing your photos you will want to start with the largest image file available, crop it as necessary then resize it to your project specifications before adding your text.
As I mentioned when discussing image formats, your usage will impact your size specifications. Generally you will want an image that is 300 pixels per inch (ppi) for commercially printed pieces, one that is 125-250 for desktop printing (refer to your user manual to determine the maximum dots per inch (dpi) your printer will produce) and somewhere around 72 to 100 for the Web.
Note: measurements for print are exact; if your photo is 300 dpi and 1 inch square, it will be printed to be exactly 1 inch square. If you print it at 72 dpi and 1 inch square it will still be exactly 1 inch, but will have less detail. Measurements for the Web are relative because they are determined by your display. On my Dell there are 77 pixels in an inch, while on my Mac there are 98 pixels in an inch. Your display may be different. As a rule of thumb I just use 72 (which was common for most monitors back in the 1990's) and keep in mind the fact that a 3 inch wide photo at 72 ppi will appear smaller on the Mac than it will on the PC. Either way it is 216 pixels, but the pixels on my PC are bigger than those on my Mac.
Consider the amount of text you are supposed to add to the image. Try to make this as brief as possible, especially if your project will be viewed online. While you may be able to use tiny type on printed matter, that type will be harder to read online. Fewer pixels mean fewer details, so 6 point type online will be tiny and jagged.
Also ask yourself if the text needs to be on the picture itself or if it can be read as a caption underneath the image. If the project is for the Web you can include captions underneath a photo in the text rather than in the image. For situations where that won't work, such as HTML e-mail, just be sure to repeat your caption text in the alt tag of the image.

For this example I'll walk you through the steps used to caption the image used in this entry. We'll add text on top of the image and below.
Captioning images can add value, but will also pose accessibility challenges. If your caption is short, you should copy it into the alt tag of your image. This will make it available to those who use screen readers or other user agents that don't show images. If your caption is too long, you may also want to link to an alternative copy of the text, either on the same page, as a footnote, or wherever you deem appropriate. Read Andy Clarke's article, Accessible alternatives, to learn more about these techniques. For this example I've linked to a description of the image and text and placed it here on the page:
Photograph of part of a sculpture featuring a man holding an umbrella next to a dog whose nose is pointed at the mans's knee. Captions built into the image read as follows:
As you've seen, it's pretty easy to add text to an image. The tricky part is making it look right and ensuring that the content is available to all. But with a bit of experimentation you can accomplish both tasks.
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