The icons should be Windows ICO files with (at minimum) the following sizes and color depths:
16x16, 8-bit color
24x24, 8-bit color
32x32, 8-bit color
If you look in the "bitmaps" folder of your SlickEdit installation, you will find all the ICO files that we use. We also include 32-bit color versions in all sizes, though we currently only use the 256 color versions.
The final requirement for resizing is that the name of your icon start with "bb" (for button bar).