I have I have tried to upload a picture twice and have had the same error happen both times.
After sitting for a little bit waiting for the image to upload a message pops up saying that for security reasons frames are not allowed. After hitting OK, the stackexchange site brings up an error message saying something went wrong and it has been reported to the developers and to report any details here.
Let me know if there are any other details that would be helpful to provide.
Note, I was able to get the images to upload after compressing them. Not sure if it is related or not.