The size doesn't have to be exactly 500x200. As long as it's no larger than those dimensions.
I know this new rule seems harsh, but it's been put in place for a good reason. Some signatures are just WAY too big and not only take up too much room but can take a long time to load for people with dial-up (and yes there are many who still have it and will have it for a long time to come).
Sorry if it ruins your picture, that was never the intention, but you can always make a new one.
For everyone struggling with changing the size of your image....try using the 'constrain proportions' in the image size area in your graphics program.
If you can't do that, don't have a graphics program or just don't know how. I'd be happy to change it for you. Just PM me and I'll do it.
Cass.