Joan,
Upps, Photoshop is a very good software, but all for our purpose here, the Paintshop Pro is the best and more favorable program.
Download the 30-day demo at
www.jasc.com. This one has very good half-automatic functions to make better a "bad" photo.
It is cheating
but I think even the Old Masters would be using PC or Mac today...
I try to avoid from using it, but this underpainting stuff is just not so easily, isn't?
The photo is too tiny to see details.BTW, reflected light, you don't need to paint it in the underpainting. Details, finesse, like at the direct painting, comes at the end.
Have fun with your "Carravagio"
PS: Ich wohne in Stuttgart.