CCK - Images fields - Overlay Image A on Image B

I'm not sure this is the right place for this, but I need help. I created a node type with two images fields. I would like to overlay field_image_a on top of field_image_b. I created a watermark preset, but the we have to specefy one image file. Every nodes produce with this node type will have different images. With the help of filefield tokens, every images uploaded is renamed a-[node:nid].png and b-[node:nid].png.

How could this be achieve ? With tokens, php or node.tpl.php ?

Thank you !