I couldn't see an existing thread for this, so I thought I'd start a thread for anyone who's created their own VMU icons!
I'll kick things off with a few I've created. I thought it would be a fun idea to have each of my VMUs themed to a specific Sega game (usually with that game's save located on the same VMU). Thus far I've created icons for ChuChu Rocket!, Sega Rally 2 and Sonic Adventure, and I have a couple more on the way for The House of the Dead 2 and OutRun. I've attached each of these files so that you can download them to your own VMU (I usually do this through an SD card and Dreamshell)!
ChuChu Rocket!
Sega Rally 1
Sega Rally 2
Sonic Adventure 1
Sonic Adventure 2
It's also worth noting that VMU icons are sized at 32 x 32 pixels, the same size as VMU save file icons. This makes the save file icons a great template for making your own VMU icons (my ChuChu Rocket! and second Sonic icons used save icons as a basis). This great GitHub page has a repository of memory card icons you can use as templates.