It's been weird for me too. I was trying to add an image a couple weeks ago, it was an imgur link, and whenever I clicked on 'update', not only would it not work, but it actually broke the whole site for a long time, like 30 minutes or so. Only for me, I think. But still.
I forgot you could upload images directly here so trying it again now, it seemed to work, until I saw that the signature now looks empty. Going back to edit it, turns out it was replaced by some massive code. Will try a couple other things, I guess.
That was exactly what happened to me. The file size is the issue it seems.