The best photoshop program is photoshop.
if you are on a budget, buy photoshop elements.
if you want something free "gimp" is highly recommended, but I use photoshop. From this point on, i'll be refering to photoshop.
I know that in photoshop (not sure if the function is in elements), you can record a set of commands. You can record the commands needed to set a watermark and everything else you want, then recall it in a simple step.
for example:
recorded command: new layer, intesert photo.
Then all you need to do, is move and resize the layer until it fits your photo.