Cannot change Signature or add a Signature picture

Okay , so every time i try to change something with my signature and then click save it says invalid file and then i try to add a picture it says invalid file again!! Someone please explain this to me like..

Isn't your sig " ~QueenTasha "?

i think its glitched you can't see it but we can.
if the file is invalid the picture either to big or to small

i wanted to change it and okay , i'll see if i can resize the picture

Yeah, I think the add/design signature only works for the new theme for the Forums but we're using the old theme so that's why I think it's not working, because of the old theme.