1) Find your image; I'll start with this one (used without permission):
2) Crop out the part you want and open the cropped image with Inkscape (see The Tools below).
3) Zoom in and then 'Select' your image. Then open the "Path > Trace Bitmap" window, update and hit OK.
4) Delete the original image from behind the new bitmap, the result should look like this:
5) Save the new bitmap image, "File > Export Bitmap"; be sure to increase the size to at least 300px and give the file a name.
Now you have a basic, 2-dimensional image to work from. Later you can add more color if you want but this is a good starting point.
I still need to go over making the basic tile, adding shadows to the tile, and finally adding your new splash image to the tile and fitting it to the floor. Most of this stuff you folks have probably already figured out and have even better methods but this is for those that don't. Stay tuned for more!
The Tools (all free/open-source):
Pyromancers Dungeon Painter
Coming Soon - Part 2 (creating the basic tile)