Now, my problem:
I've just been sat here happily uploading some gallery avatars to my forum & when I went to test them out I started seeing this:

It doesn't just happen with the new avatars i've uploaded today, it does it with the existing ones which previously didn't generate this warning.
I always virus-scan stuff before I ftp it, & I have double checked everything i've uploaded today. I have quite a few avatars uploaded, all categorized into separate folders to make it easier to view them, could it be the permissions on the folders causing the problem?
I've had a look on the Cracker Tracker website but I don't speak German so that was no help.
Also, by default should it let you use bbcode/html in signatures? Only its not letting me & I wondered if that could be something to do with it too.
*Edit* I can access all the avatar folders & use them all if I do it through the ACP/Users/Manage. Its just doing it through the profile page produces the security warning.