There are many differing decal methods - see attached (my other hobby is slotcar racing hence lots of miniature decals).
This is a low-res image.
However clear decals in waterslide or vinyl do not work well on solid colours unless you first paint a white (or other light colour) as a background.
You can print solid backgrounds using a sublimation printer.
You need the correct printing papers - there is a US company Papilio (Disclaimer I have no interest in the company or its products - I have just found them to be very helpful.)
Home papilio.com official website.
They do a wide range of materials and have excellent technical suggestions.
They also do a "magic" waterslide that does not require lacquering. Normal gelatin waterslide paper is not waterproof and the inks tend to run on being wetted - so you have to "waterproof" the print by lacquering - you can also lacquer after application to further secure and protect the decal.
Gelatin waterslide paper is also generally available from print supply shops that support the novelty printing industry for T shirts, Mugs, key-chains etc.
Tip: When laquering inkjet printed waterslide do so flat and make the first coat as light as possible - otherwise you can get colour bleeding - ditto runs if the paper is not horizontal it will make "runs". Make sure that by the second or third coat the surface is completely glazed and waterproof.
As others have suggested you can also design your own artwork on various CAD or graphic packages as well as directly from photos and so on.
Regards, Ken