To add to @Reheer's comment, the image in the AirDrop dialog and the image you are using in your KM macro are visibly different, so that's why KM is not seeing it. It's not really to do with it being round versus square.
What you need to do is, when the AirDrop dialog is showing, take a screen shot of it and then crop it so you're left with just the head - something like this:
and use that in your KM macro.
Or you could even crop it like this:
and that would work too.
PS: don't use the images I've provided - you need to create your own on your own Mac!