Mail: Signatures could be sharper!

If jpegs or screenshots are used in „Mail“ as signatures, they are displayed in a rather mediocre quality. I tried several formats and sizes ... the result is always the same (= not good). If, however, you drop a screenshot directly from the finder into your email-message, then the quality is sharp and crisp. Why doesn't that work when jpegs/pngs are integrated in „Mail“-signatures?