Save as either a Jpeg, GIF, or PNG (I'm assuming Macrumors supports this format.) for the web.
Do a Save for Web... (shift + option + command + s ) in Photoshop. This will allow you to see the actual size before saving it. It will also allow you to optimize it so that you'll get the best results for the target size. It also saves a generic icon without the OS X preview so that the file's size on disk is close to what it will be when uploaded. This last tidbit isn't that important though, since the preview from a normal save gets stripped when uploading to a server.
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