there is several issues with the [Set icon...] feature:

1- I tried different .ico files at different resolution 16x16 and 24x24 but all of them didn't work without warning. There is no documentation suggesting anything about it, so is this a bug? Additionally adding an error message if possible would help new users like me.

2- I switch to .bmp, it worked but the button/icon size is too small to use for most kind of icons, and it is smaller than other icons in the toolbar:

I used a blue border to see if the image is scaled to fit, and indeed it is, so the issue is not the dimension of the .bmp file. I suggest giving the user more granule settings over the icon such as their size.

Additionally, the icon is offset to the top right area: