By using a Resource Hacker your can replace the default MSPAINT icons in MSPaint using the following template.
When you go to replace them in the Resource Hacker go to the tree menu:
+ Bitmap
Then:
+ 859
Then Click:
1033
There are your icons for the replaceing. You can use Resource Hackers such as ResHacker or PE Exploerer for this, but any Resource Hacker should work.
P.S. - The squares seem to be about 16x16 each.
Post yours so the world can see
