Well it depends what the size is after compression. Gif are normaly used for pictures that have little collor (and gif can be transparent).The compression of is some thing like this.
Example:
a picture with the following line of collors
Red Red Red Red Red Red Red Red Blue blue green green
after copression the gif file just says.
5* red , 2*blue 2*green
Ofcource if you got a picture of a person the collors will always differ from the previous pixel end the gif copression is useless. And don't forget that Gif can only use 255 collors. But the copression is lossless
JPG
Jpg uses a diffirent compression (lossy) it removes all information in the picture that the human eye wouldn't detect anyway.
In general this is the rule.
a picture with a low collor count and has lots of lines uses gif. If it has lots of collors use jpg. (icons are aways Gif )
Personally I will stat using the PNG format. <img src="/ubbthreads/images/graemlins/alien.gif" alt="" />
here 
