There may be a shortcut if the forum has something built in, but you probably need to do it offline/separate to the forum in some sort of editing software first before uploading them. Some viewing software will correct the image but it's just for viewing, it does not actually save the new settings. Make sure that you correct the orientation and save the setting, then upload.
Alternatively you can use an online tool like this http://www192.lunapic.com/editor/?action=rotate
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