Without seeing the images/forms you're talking about, I can't render an real opinion other than this: Much of writing Alt Text (or electing to artifact instead) is subjective. We have to ask ourselves: is this image actually presenting information to a sighted user that would be hidden from someone using non-visual reading means? If the answer is "yes", you must provide equivalent information (in the Alt Text, around the image, in a longer description, at a URL, etcetera). If the answer is "no," then artifact the image. You can always test by using a screenreader on your document yourself, with your eyes closed, and decide if everything makes sense to your ear and mind. Acrobat has a built-in screenreader, or you could install JAWS (paid) or NVDA (free), which are Windows-only. If you're on a Mac, use the built-in Acrobat screenreader or the Apple device's built-in VoiceOver utility. If you're really not sure whether to artifact (maybe you're too close to the subject or too deep into the document to get that objective [heh] subjective perspective), ask a co-worker or friend if the pictures of the forms help them in any way. Also--perhaps ideally--you could hyperlink the images of the forms to the correct forms, which would help sighted users with the thumbnails and anyone else by providing a direct link to the correct form they want.
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