Cannot upload "Custom image" in forum customization screen

I tried to upload several custom images, both in Safari and Chrome and it didn't work.

After the upload, the newly uploaded image is not used. The old one is used instead.

Here's a screencast: https://www.dropbox.com/s/2w7o2tfr9f3tm1p/UserEcho...

I solved the problem for the moment by using the "Forum Image" instead of a "Custom Image".