Okay...
Make the image the size you want your monster to be.
Open the tile set you wish to rip from in another window.
Now at the top, select a 32x32 grid, and choose snap.
Copy the one square you want to be the sand.
Close the tileset, don't save changes.
On the monster image, follow the same grid instructions as above.
Paste the sand tile and move it on the different sqaures until the whole image is covered.
Now under the layer properties, turn opacity to zero.
Make a new layer, and select it so that it is your current one.
Draw the outline of your monster.
paste a color that is different from the outline outside of the outline.
Under layer properties, select the area inside of the outline as transparent.
Go to the sand layer and make it's opacity up to full, 255.
Merge the layers and delete the area outside the outline.
You should have the monster filled with the sand. It only takes me 3 minutes to do this... If you need images along with the text, let me know.