Seeing the same issue this morning, sure it was working quite recently though.
EDIT - yep, posted a gif 4 days ago and an image a week ago, so something has changed since then.
EDIT 2 - trying that same image again:
And that seems to work. Filesize maybe? Or extension (my first attempt today was a jpg, that one is a png)?
EDIT 3 - jpg, 1.5Mb:
Success. That's a smaller version of an image that failed to post. Bigger version was 2.9Mb, so maybe there's a limit?
In case it helps...
Device information
Sync version: v24.03.26-14:56
Sync flavor: googlePlay
View type: Smaller cards
Push enabled: false
Device: raven
Model: Google Pixel 6 Pro
Android: 14