you want to look at the ICON in dialogs
you may have to color the background in mspaint manually to get the grey, also note that mspaint will use the upper left corner color as a transparent background.
sample
Code:
alias icon dialog -m icon icon
dialog icon {
  title "Icon Test"
  size -1 -1 100 50
  option dbu
  icon c:\windows\notepad.exe, index
  icon 100, 0 0 48 48,"c:\windows\blue lace 16.bmp", noborder
  icon 101, 50 0 48 48,"c:\windows\blue lace 16.bmp", noborder
}
on *:dialog:icon:sclick:101: { run mspaint "c:\windows\blue lace 16.bmp" }