I used the Modelview.exe, I put the item I wanted to screenshot in the right position, and made two screenshots. One where the light is set so that the item is completely black, and the background white. This shot will serve as an alpha mask for Photoshop (Also disable bloom, else it may not work.) . In the second the background is black again and all effects enabled, default lights are on again, this is the exact image I use for the alpha mask. So both images must be on the exact same spot. And of course, in both shots the floor is disabled.
And last but not least I used Axialis Iconworkshop to convert them to *.ico.