Max Keyboard requires all text be converted to paths before uploading. This can be done in inkscape:
- Select any letter
- Edit -> Select Same -> Object Type (This will select all text)
- Path -> Object to path (This might take a while)
Two copies of the design are included, one with text, and one with paths.
Note that the template provided by Max Keyboard has several layers. Be sure to edit the "Your Design Layer". You can make the "Example Layout" layer to see a standard key cap design.
This design was done for a backlit Corsair keyboard. The backlight is on the top half of each key, the bottom half is barely lit. That's why the primary letter is placed on the top half and the shifted portion of the letter is placed on the bottom half and appears darker. The keys with designs are shurnk to half height so that they don't appear bright on top and dark on bottom. It's probably desirable to adjust positioning and size for a non-backlit keyboard.
This design is also a Dvorak design, the key positions are switched around and extra QWERTY markers are placed in the corners of each key. It shouldn't be too difficult to switch things back. It's also for a keyboard with a "6.5" length spacebar. If your space bar is a different length, you'll probably need to download a different template and copy the design layer over to it.
http://www.maxkeyboard.com/mechanical-keycap-layout-and-size-chart.html
There are of course several more keys where Disney/Pixar designs can be placed, such as the function keys and the keys above the arrow keys (scroll lock, home, end, etc). I find a google image search for Disney silhoutte to be the best way to find appropriate artwork. Inkscape has a trace bitmap feature that can be used to create a vector art path out of an image. Be sure to drop the number of colors down to 2 in the dialog.