Sorry for starting with a negative but I have just installed Zen Photo and what strikes me immediately is the image rotation option is horrible. You can rotate 0, 90, 180, 270 degrees. I understand that but from which corner are you rotating and in which direction?
Also when you have done this, is the picture re-saved at this rotation? (It would appear not, which makes things confusing).
A preview is in order. flickr added this and it is brilliant.

Positive values mean always clockwise... Images are saved in the cache, so yes. The full image (the original one you uploaded) is generally not touched.