The issue I am seeing is occurring after an update from SharePoint 2010 to SharePoint 2013. The library is a publishing library created in 2010. These features are enabled in 2013. Looking at the "Thumbnail Preview" column in any view shows the generic sheet of paper icon. Viewing the images in the "Thumbnail" view shows the thumbnails correctly. The images for the list in question are all .gif. As far as I know those are the only ones failing, but we could have missed some other filetypes.
I've tried moving a document to a newly created Images library and the preview shows as expected. So I believe I've pretty easily demonstrated that the actual image is not a problem and that SharePoint 2013 can render it. However, it's not a workable solution to move these files.
If I open the library with Fiddler, I get a 404 error for 256_icgif.gif. From there it's been sort of a dead end as I wouldn't know where to test and see if that's part of the issue.
This is our dev environment before upgrading to 2013 so we're looking for a fix we can perform during or before the upgrade.
Thanks!