Ref T10262. Currently, we always render a tag like this when you {F123} an image in remarkup:
<img src="/xform/preview/abcdef/" />
This either generates the preview or redirects to an existing preview. This is a good behavior in general, because the preview may take a while to generate and we don't want to wait for it to generate on the server side.
However, this flickers a lot in Safari. We might be able to cache this, but we really shouldn't, since the preview URI isn't a legitimately stable/permanent one.
Instead, do a (cheap) server-side check to see if the preview already exists. If it does, return a direct URI. This gives us a stable thumbnail in Safari.