First you need to install PowerIcons or (better, but can be fiddly to set up) AfA_OS. Once one of those is installed your Amiga will be capable of displaying PNG icons. This is just a PNG file renamed to have a .info extension, so if you want you can just get a PNG icon you like and rename it to .info, and it will show up as an icon on the Amiga. Ken's Icons contains a whole load of PNG icons with the extension already changed to .info. Your sig says you are using OS3.9, so you can just click on an existing icon, bring up the Information, then drag the new icon over the old one inside the Information window, and the image will be copied.
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moto