The original bitmaps are those with names like "s_b.bm" and "ol_b.bm"; 
that is, the 80x80 set, solid and outline.

The 64x64 set and 40x40 set were derived by scaling the 80x80 set and
touching up the results by hand with a bitmap editor.  Some of them
could use further touchup.  The 64x64 bitmaps are in files with an "sm_"
prefix (for "small"); the 40x40 in files with an "xs_" prefix (for
"extra small").