On the Photo Albums -> Album admin -> Edit -> Manage Photos – Edit photo information screen at every photo there is a row of buttons:
Rotate left, Rotate 180, Rotate right, Flip–, Flip | …
If you want to rotate by other angles – eg to correct a shallow horizon -, you can do so in small steps if Image Magick is installed on your server. See this ducumentation page for more info on Image Magick.