Not TOO big!
Marty,
100K is the max the The Gauge system will let you do. If you have posted the pics elsewhere, or want to "point the system" to a pic, say on another website, for instance, you can "link" to it using the "IMG" button above the box you type your text into, and the system wiil fetch the pic from that website and display it with the post. That "linked to" pics can be any size you like. However, there is a practical limit to the size of pics. 30K - 55K JPG format files for pics is about as big as you can pratically cope with. A 500K pic will give no better resolution on the net, than a 50K one, plus it will take 5 mins or so for it to download so people can see it! (on dialup).
75 DPI resolution is good for the net. higher resolutions are for photo quality printing.
Happy pic posting!