It’s actually been flaky for the past couple weeks, but up until today, just repeating the upload a few times gets it to work.
Same here with re-trying a submission until it works, except the last couple days I've had the same issue with Lemmy.World uploads. (wow, and I just noticed they're still running v0.19.3)
I think what might work in cases like this is using Imgur for the URL field, then manually adding a thumbnail as well (a newish feature). EDIT: Nah, the "thumbnail" field seems to make no difference, at least sourced from Imgur.
Or possibly using other sites (such as Lemmy instances), users won't need to click the thumbnail / link to see the image content.