Any graphics program should allow resizing of images.
You could also use imagehack to to reduce size in the setup dialogue.
I would be surprised if converters also allowing size manipulation are not available online (-> Google).
There has been a size limit for signature pictures introduced in the past, years ago, but neither people nor moderators cared for it once the limit was announced. So here we are.
I would say a good rule of thumb is that a signature image should not be greater in size than a message of two or three sentences that you post. But that is just my opinion. However, some people are exaggerating it, I think.
As long as you haven't fixed the size and still figure how to do it, I would recommend to temporarily disable the display in your personal options. You could activbate it again once you got the correct image to use. - And btw - why TWO graphics?
