Right, so lets say you have a pic off the web thats a non copyright image, CC or otherwise able to rip. Its RGB like a JPEG image and you want to make it into a PNG token with a cut out shape. Heres an example. HiRes pics from here http://grin.hq.nasa.gov/subject.html but I am using smaller versioned ones for examples.
You could do this in a number of ways but this is the way I use. Starting with the X36 plane pic, look for a color that is definitely not in the image - like bright red. Get the single line tool and draw around it. What I tend to do is put the start point down near to where I want to start but a little away from the edge I am trying to follow. Then drag out the line and put the end point down exactly where I need it. Then without moving the mouse, start a new line and drag back to the starting point again but this time set the end point down exactly where it needs to be. The reason I do it this way is that you dont have to guess the angle exactly in one go. Like image below but end point 3 past the corner. Note, turn off anti aliasing for the line. Let it have jaggy edges. Otherwise it will blend red into the image and we will want to pick up the red in a mo.