From MXCL, safe to a lossless format like PNG. Open the file in Photoshop, edit it as desired and then save it via
File >
Save for Web & Devices.
In the dialogue, play with the
Quality setting to get a good trade-off between looks and image size. Since we all have broadband by now, i don't see any reason to be overly cautious with filesize. We're dealing with artwork here, so details matter.
80 or
85 should put you on the safe side.
Save and upload to an image service (or your own webspace), that doesn't recompress the image. I've lately been using
tinypic.com to much satisfaction.