Patrick van Arkel (Vampier) released a SCREEN 5 Graphic Editor, which allows to create 16x16 tiles. Vampier used to use a similar program in the past, but after it was lost somewhere in his disk archive, he decided to redo the whole thing. I've been helping him out with a bit of assembly to speed up the things that turned out way too slow in BASIC.

Direct download link: Graphic Coder