The total size for the signature needs to fit within the allowed size. This does not just mean that the image is that size. It refers to the entire signature, which means the text of the signature too. The default size of font (unless specific fonts are specified in the sig) is about 20 pixels high.
Your overall signature is much larger than the 600x120 pixel allowance. 255 characters is the character limit, but there is a pixel dimensions limit too. That had been enforced until about September of last year, until a template file got damaged breaking the check that had been in place for years.
I've just updated the text where signatures are edited to make sure to specify the pixel limitations. I think an update to the software may have lost some of the stuff I had in place.