I found a couple other threads on this topic, but they were all looking for ones other people made. I know this is very simple, but I figured I'd post it anyway as I spent a few hours looking for premade ones with the subject matter I wanted, before realizing just how easy it is to do it myself.
What you need:
Clear packing tape
A printer with ordinary paper
An image you like
I picked this picture:
Tauren Druid by Cadychan
All you need to do is crop the part of the picture you want; MS Paint does this job fine. Generally, the cropped portion needs to be about twice as wide as it is high, but that may vary on the diaper and personal taste.
After doing that you should end up with something like this:
The white space is there so the actual picture will maintain its proper size when printed to fit a page. Some programs will cut out the white space, however, and I'd recommend using a "dumber" program like MS Paint to print it. Feel free to use that image as a template. If you decide to use it, make sure to print in Landscape page orientation.
Once printed, cut it out and stick it to the diaper with clear packing tape. It's important to cover the whole picture with tape, so it bends properly with the diaper.
The template I provided is designed to fit medium size Bambino diapers. I don't know if other size diapers have different tape area margins. Printed out and affixed, you get this: (Sorry for the low quality, all I have is a webcam)
The diaper is now diamonds! Well, sort of.
Hope this helped someone.