Yep - my "diarrhea on a meadow" was, despite initial impressions, very constructive as well. ^^
I mean - what else does this message convey if not "the colors don't do together very well"...
A reason why the blood looks more like, uhm, you know what, is that the green background changes your color perception. It kind of "drains away" the color of the foreground graphics...
I just made a small picture demonstrating how your foreground color looks different on different backgrounds.
(Notice how some combinations look better than others and how the foreground color seems to differ although technically it stays the same.)
If you really want to design a nice image (or a website, for that matter), you really should read something about color theory since it's neglected FAR too often:
http://www.worqx.com/color/
The "P.S." remark should be interpreted as: make your signature smaller. ;-)
P.S. I'd also suggest NOT saving your signature as a .jpg since this reduces its quality. Signatures are usually downloaded only once and are kept in the browser's cache ever after so the higher file size doesn't matter. The hosting site doesn't use more upload bandwidth and modern connections can handle such small images very quickly anyway. I'm not sure if the alternate method of lossless compression used in PNGs wouldn't even result in a SMALLER file!