The GIMP is open source and all, but I eventually settled on a 2nd-hand copy of Photoshop from the Goodwill Computer Store for $2-4.00.
This mock-up — who is the audience? Blown up to poster size for a room full of people? 4000x6000 pixels?
Here is the method I use:
- the photo
- cut and shut in Photoshop
- a tracing on thin paper laid over a printout, scanned back in
- A final beauty pass in Photoshop
The next step could have details bleed through from a lower layer