AdiIRC

As you can see in #3693 and #3678, [x y w h] is a required parameter for non-icon images.

Example: /toolbar -a GRbug GRbug blabla.png 0 0 16 16 "/noop"

hmm it seems mIRC is limiting supported images to some kind of aspect ratio or more likely by max dimensions. Try attached images.

/toolbar -a test test fail.png "/noop"

/toolbar -a test test works.png "/noop"

It seems to be a max of 440 pixels in either direction, but there is also some aspect ratio involved, I made it a flat 440x440 before [x y w h] is required.