Is it possible to rotate images in Preview in degrees other than 90?
If you're not averse to using the command line, you can use the sips command to rotate images. For example…

sips -r 23 --padColor FFFFFF image.jpg


…will rotate image.jpg 23 degrees clockwise and "fill in" the empty space with white. (If you rotate PNGs, the alpha channel should be retained.)

Worth reading the man page for sips as it can do other useful stuff.