I would like to add the following. Hope you don't mind Geki.
Save for Web
I have reasons to believe some people have never used the "Save for Web" option... Browsing the forum, and checking on sigs. I see quite some people using png-8 and gif for no apparent reason.
Hence a mini guide.
The save for web option
After clicking it you see the following screen
1. This is where you choose what format. You can choose between the following
Jpeg
Gif
PNG-8
PNG-24
WMBP
But as a sig maker you won't need anything else than jpeg and gif. You won't need anything else but Jpeg if you don't make animated stuff and don't mind sacrificing some flexibility.
2. This is where you specify the quality/compression of the img. If you click on the arrow you will see slider. The slider allows you to input each digit between 0-100. 0 being higher compression thus worst quality. And 100 is best. I won't bother with the gif compression since no matter how you do it. On the current img you will see quality lose. Even putting diffusion at 50 the umbrella still looks bad. And your size won't go below 40kb.
For Jpeg i recommend to put the quality at at least 60. Below you will see examples with different compression. Around 60 is were you will start seeing lose of quality.
Sig examples
A quick crop and scale sig as a example.
Gif 47.89 KB
Nice umbrella ay? The diffusion is at 100. At 50 it's less ugly. But still notable quality lose.
Jpeg (quality 100). 64.85 KB
Jpeg (quality 80). 35.2 KB
Jpeg (quality 60). 22.37 KB
Conclusion
My conclusion would be save them as a Jpeg with quality at at least 65 to be on the save side. You can see minor quality lose at the jpeg at 60. But imo it's still better than the gif. And it's only 22.37 KB. So use Jpeg for your static sigs. If you want to make good use of Transparency. A post of me above explains a good way of creating transparency effect with Jpeg. I seriously see no reason to cripple your own work by choosing png-8/gif to save them as. If you do have a good reason i really like to hear them. Perhaps i missed some hidden option on gif that makes them worth it.