The smaller in size, the better.

[imported_plattopus]

The main thing I notice when looking at postbits, is that all too often developers take the easy way out and use images instead of creative HTML. It's far easier from a design point of view to make each postbit a few images as opposed to a whole lot of HTML... even though the outcome is the same.

HTML loads many times faster than images do, so use it as often as possible!