Aye, for some reason this error pops up on occasion, even though no one is trying to hack your account. Try logging out of your account, then logging back in using the box under the forum banner, and then you should be able to change your avatar/signature without the error messages happening.
Sorry for the confusion; our forum is a little quirky at times! Your account is safe (but if you are worried about security, you can always change your password whenever you'd like).