Image in Mac Mail Signature too big!

The image I drag and drop into my Mac Mail signature looks good at first, but then grows huge when I compose an email or go back to the signature edit window. Other entries suggest "scaling" but when I do that I loose the detailed resolution. I pulled the image from a received email that looks perfect in Mac Mail, but just does not behave when putting it in my signature.

How do I fix this without having to create an html signature?