I just registered and tried your site
I don't see any posts at all in the gallery.
So I am back to you need to verify what graphics library you are using and set it correctly.
Normally GD2 is by default by your host.
So set it to that at least to start with in
Control Panel » Gallery Settings
Or you can run php info and figure out what you have.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers