Hey Jimmy. They don't have to be real small as the forum software will resize them, but it is best to keep them small to ease the upload. I've uploaded 1024x768 & larger, but had the best luck with 800x600.
If you're putting a pic in a post, whether you've done a Post Reply or a Quick Reply using the Go Advanced button, above the Message box click on the little paperclip icon (next to the smiley face) to open the attachments window. Choose File or Browse (depending on your internet browser) to search your computer for a photo. Click on Upload. Close the attachments window & then click Submit Reply to make your post. The pic will appear immediately with your post.
If you're just adding a photo to the photos section, you'd click on the Upload Photos, Choose File or Browse, find a photo on your computer, click on Upload/Submit & your pic will immediately show in the photo gallery (or in whatever gallery you chose to put it in).
It can seem kinda complicated at first, but once you've done it a few times, it gets easier.