Why should images be optimized?
Search engine spiders can’t see images. They pass right over them when crawling a page. This is a problem because images often contribute to a page’s message, sometimes even convey most of the meaning. If a crawler comes across a picture, it won’t be read or indexed unless it is properly optimized.
Correctly optimizing images can improve a site’s SEO ratings and increase a page’s chances of being found through the images on the page. Optimizing images is also helpful for people who are using screen readers or have slow connections and have turned off image viewing in their browser.
Search engines can only read text. In that case, if we want images to be crawled, they must be translated into text. ALT (alternative) tags are like nametags for pictures so that crawlers can understand the images. ALT tags can be inserted in the HTML of a page. They look like this:
An ALT tag should be a key phrase describing the picture. It shouldn’t be a default IMG_008.jpg, nor should it be an obscure reference such as "library_picture1." Make sure everyone can understand what the image is without seeing it. A good practice is to include two keywords and one short phrase describing the picture. For example, "ISU students in library with coffee." Avoid keyword loading: "ISU-fun-event-books-exciting-freshman-torch-fire-signing"
Filenames and Captions
Filenames have a similar purpose to ALT tags. Crawlers can see both the filename and the ALT tag in the HTML of a site. The filename shouldn't be exactly the same as the ALT tag, since it will probably be shorter. Filenames need not be longer than two or three words, preferably a keyword and descriptor, "ISU-library-students.jpg."
Captions serve a slightly different function. Neither the ALT tag nor the filename are visible on the page, but a caption is displayed. A caption should succinctly explain the picture for readers. A caption should not say, "This photo, taken last Thursday, depicts two students interacting over coffee in the library." Instead, a good caption would say, "Two students enjoy coffee in the Cunningham Memorial Library."
A note on image formats
Search engines have difficulty displaying some types of images, such as X & Y. Before uploading images, make sure they are in a universal format, such as .jpg, .gif, or .png (make sure the extension matches the file type). For instructions on changing image file formats, visit xxx.com.
ISU Best Practice checklist
Format (ALT): Keyword1 Keyword2 Descriptor Phrase
Length: 2-6 words
Content: keyword and phrase describing the photo
Appearance (HTML): Image tag: img src="file-name.jpg" alt="good alt text for SEO" height="xx" width="xx"/