Just before you upload it to Photobucket - click "resize image to..." Choose a well sized option and copy the code fromm the Direct Link. I sure hope this is what you mean. EDIT: The featured map image sizes vary - but the most recent ones were 640px × 480px. Alternitively, edit the size by clicking the 'Edit' button after it has been uploaded.
http://www.forgehub.com/forum/halo-forge-maps/18644-embed-screenshots-your-map-thread.html Follow this guide ALL the way to the end. You probably know all of this but follow it very, very carefully and every single step right till the end.