Mac: Combine JPEGs

Open both images in separate Preview windows. Go ahead and save one of them as the name of the combined file you want to use (you definitely don't want to overwrite either part until you have the combined image you want).

In the renamed picture window, Edit/Select All, Edit/Copy, then Tools/Adjust Size... and make the image large enough to hold both parts. Don't worry if it's too big, you can crop it later. When it's big enough the image will be stretched, possibly weirdly if you didn't scale proportionately, but it doesn't matter because you're going to overwrite that image anyway.

Edit/Paste and with your mouse, drag this image to where you will want it on the final image. When you have it placed where you want, Click somewhere outside of that part on the now stretched part of the image, which will lock the pasted part where you have placed it.

Go to the window of the other part you will be adding, and Edit/Select All, Edit/Copy it, then click on the image you will be pasting to, and Edit/Paste, move this part where you want it, and again click on some part of the image outside of what you've just pasted and moved to lock that in place. If you don't do the clicking thing, the paste will not take.

If you need to trim parts of the image, Edit/Select All to select the whole image and move the dotted lines with your mouse and the little circle handles until everything you want is in the lines and everything you don't want is outside. Now Tools/Crop. You may need to redo these steps as sometimes a small bit you didn't want will be included.

If you make a mistake, you can Edit/Undo, so don't worry about making mistakes.

When you're satisfied, just File/Save, and voila, you've joined two images into one.

From: http://forums.macrumors.com/showthread.php?t=344576