I'm not sure if I've understood what you want to achieve Mark but if it really is drawing on top of a photograph you could try Gimp. OSX version here:
http://gimp.lisanet.de/Website/News/News.html
Thanks for that Peter - I found itfar more complex than I need and have settled on Sketch which, although its acquisition by Evernote and subsequent 'improvements' suits my purpose well. It is particularly good on an iPad.
Mark
Mark Ford 