Seems like it might be due to file size. Anything more than a few hundred kilobytes fails to upload.
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Yeah ok this was indeed the solution, compressed the image down to ~600KB and it uploaded fine.
Got same error, solved by reducing size. The error message would need some improvement, though...
Yeah it's a file size issue. I had the same problem trying to upload a gif.
Haven't tried uploading an image today, had same error as you, didn't work on jerboa, did work on mobile site.
Will try again later (when I have something to post)
Ah ok. I shall attempt some image compression and see if that works shortly.
Yeah, I was having that issue. Now that I read the comments here I realized that I was trying to upload a 3200x3200 image, which is pretty unnecessary for a picture of a pizza. Changed it to 1000x1000 and it went through with no problem.
This is the way.
@[email protected] was saying a while ago that storage is gonna be a problem. Why not just link your image instead of uploading it? Format is 
Tell your instance admin to increase the maximum file size.