everything with more than four pictures in one post (because Mastodon has to convert pictures in a post to file attachments, and it can only attach four files to one post)
actually, everything with more than one picture in one post (because the order of the pictures may be reversed, depending on where the post comes from)
everything which relies on pictures embedded in the text in specific places (because those pictures converted to file attachments are attached below the post)
Technically, you may be able to view them. But chances are big that Mastodon will mangle them server-side, independently from which app you use on your phone.