Mixed Media Question

Hi all,

I'm still quite new to the Gimp. I'm an artist and would like to start doing some mixed media art. I actually want to merge my photography with my drawings.
Examples of two images I want to merge:

http://kabai33.redbubble.com/works/1769118-3-snail-spirit

http://kabai33.redbubble.com/sets/13537/works/1688020-4-mandala-flower

 Now, when I put the mandala over the snail I also want it to look like I drew it on the snail itself, so what I need to know is if this is possible and how. Do I need a plugin? Or how else can I create this effect.

Any help is very much appreciated!