I've found that chips displayed at 1 1/2 x actual size are about right for this board and for eBay auctions. I've played with jpeg compression and found that a file size from 25K to 50K looks just fine... any more bits are lost when the pic is displayed and just slow down the loading of the page for those using dial-up connections.. I usually use a JPEG quality factor of 5 (on a scale of 10) for my scans. I use Photoshop LE for picture editing.

You can compress a lot more that this and still get OK images. Lots of the guys post with file sizes of around 4K-5K. That's about what Greg's chip images are in the chip library, but he uses a smaller image size. JPEG is pretty efficient.