Thanks for the info in this thread.
I struggled to find the Avery 18665/8665 in Australia and eventually just got this from ebay for a whole AUD$6.05 delivered:
"7 x A4 CLEAR INKJET PRINTER TRANSPARENCY FILM SHEETS TRANSPARENT ADHESIVE LABELS" (eBay item number: 360473271273)
I wasted one by printing on the wrong side, and another by not using the correct settings on my elderly Epson RX650 (had to turn off high speed bi-directional printing). The adhesive appears to be like what you'd get on vinyl decals, so you have some leeway to lift it off and adjust placement.
As you can see from the gallery link below these buttons have notches in the plunger so the diffuser doesn't move. I have a different variety of buttons e.g. white/coloured/chrome bezel, with either white/coloured led and/or clear/coloured cap. I decided on just plain black images with any colour combos, and coloured images work best with white/white/clear. You can see one of the first buttons with Mario is all washed out yellow. That is actually a white led, white diffuser, clear button cap, it is just the bezel that's yellow. Looked great until turned on the led, made it all yellow. Yurk.
The results seem okay for me. These are just some experiments:
http://imgur.com/a/KclEYWould love to see some gallery's / links to others for inspiration. Some of the images turned out a bit pixelated so may change them or just accept them as being retro.