I don't know if this is a Mac thing or a FireFox thing (can someone confirm?), but using the image that you posted up above, Katie, I could click and drag it next to my signature and see if it was the right size. I've included a screenshot below.
Steps to test:
1) Start a new thread/reply. It doesn't matter where, you wont be posting it.
2) Include that image.
3) Click 'Preview Post'
4) Drag/drop the image next to your signature.
5) Adjust as necessary.
