Depending what pictures format... Usually pictures are stored in memory like a very long string where 4 bytes represent one pixel. So you might either rotate the picture by reordering its data in memory, i.e. going picWidth-times linewise from "top to bottom", pick 4 bytes and rearrange them from left to right.
If just for display purpose you could simply draw the picture onto a mesh, i.e. use it as a texture like in the example shown here: http://processing.org/learning/p3d/
but "rotate" the UV-values (check the code near the picture of Berlin-tv-Tower on linked page).
Rotate UV-Values= these are the last 2 parameters inside the vertex-definition . So do not really "rotate" them by some matrix but exchanging the values in a circular way.
Instead of rotating UV you surely can rotate the mesh also with the same result.

Simplest idea: rotate the image using a common drawing app like paintshop

I got it, i messed up 2 lines which should be:

Open the image in a good image viewer or editor.
Use the software to rotate the image and to save it.
Problem solved, no need to do that in code...

people only read the topic title? :D

Yes...
(Or, sometime, I tend to scan quickly the text.)
Though I re-read your explanations and don't see more information than in the title, actually.