Thanks Greenback.
They're waterslide transfers. You can get the paper for them off eBay for a few quid, and print them using any desktop printer.
This is the stuff I used:
https://www.ebay.co.uk/itm/232322269621?var=531574309332
Then all you need is a quick covering of spray varnish/lacquer to seal the ink before you wet them. I'm not sure whether they'd be any good for a bike. They'd definitely need a good strong coat of clear lacquer over them as they can be quite fragile. Depending on the printer, they might fade in sunlight. Not sure, 2D is not really my area of expertise.
If you're after some decals doing, the member GilM on here seems to be everyone's go-to guy.