Pop-Art tutorial in GIMP
get image
get/make a palette, preferably one that matches your image a bit
grow your image
set color to indexed
click "use custom palette" and click the palette you wish to use
now your image is very pop-arty, but also very sharp; no antialiasing.
fix this by setting the color back to RGB.....
...and shrinking it.
final product (mine looked a little bit weird; i shrinked too much)
if you have any trouble, that's your problem