The GIMP is a free version of Photoshop, and it also has a step learning curve. I thought other Linux, or Windows and Mac users who can't afford PS, might find this tutorial informative.
Also here is a generic tutorial for using Layer's in The GIMP, -> The GIMP Tutorial - Using Layer's to Create Space Art
In addition to that, Eric informed me that he posted in another thread his steps to invert drawings, that can be seen here: Eric's Tutorial
Goal, take sketch and invert colors, adjust brightness to have skecth appear as view through telescope eyepiece.
Using cildarth's M13 image, open the file in GIMP.