Sure.
If you mean adding pictures to homebrew (stuff that's in the Homebrew Channel), all you need is a 128x48 PNG file called "icon.png" in the folder the homebrew uses (sd://apps/appnamehere/)
If you mean adding pictures to games inside Ultimate USB loader,
those will actually automatically download if your Ultimate USB Loader has internet access!
But if you want do it manually, that's super easy too!
Go here
Check the checkboxes next to every game you want.
Click "Zip and prepare download" at the bottom.
You want to unzip the zip and put it in a folder called sd://images/ for 4:3 tvs or sd://wimages for widescreen tvs. I can't remember if the zip and prepare download function automatically does the folder structure or not.
More Ultimate USB Loader help:
You might also find that when you rip your games that they aren't named properly. Perhaps Wii Play is named "PARTY PACK FOR REVOLUTION" or something stupid like that. This can be easily fixed.
Go here to download titles.txt. Put it in your USBLoader folder (sd://apps/usbloader/ or whatever you ended up calling it). Ta-da! Problem solved!
Best of luck getting your Wii working wirelessly or getting the USB adapter working properly!
I can integrate this stuff into the for dummies tutorial tomorrow.