I see your sig is a PNG. These should be reserved for renders/abstracts with a transparent background since they are notoriously huge in 24 bit color format. Try using a JPEG. Good quality and file size. The 'save for web' function gives you the option of setting the optimization from 0-100 or 1-100 (I forgot). I usually optimize at a range from 65-75. 100 is overkill and doesn't show much of an improvement when comparing to a 75 or 80. Using a GIF is also an option to lower file size, but it only allows up to 256 colors while PNG-24 and JPEG go up to 16.7 million, methinks. GIFs should be only used for animations.
indeed, try saving your work in a "JPEG - jpg" format if your looking for good colours viewed on webpages, save as a "GIF" but yeah png isnt the suitable format for sigs