Buy the model makers decal paper from Testors and a small rattle can of Krylon acrylic clear.

Grab images and art from online.

Not necessarily a need to get fancy or convert, JPEG work. Can use the most primitive Paint program to clean it up, resize, color edit etc..

Use appropriate settings and use an inkjet printer.

Allow ink to dry. Next is to lightly spray coat with clear. Two fine coats is ok. Let it cure and set for a few days, room temp no humidity.

Trim perimeter of art. Handle with a flat nosed tweezer. Follow the routine in applying as to a model. Once cured and dry, Testors has a formula called Decal Set to top coat. Or just spray a small amount of Krylon clear in a container and while in liquid state, dip a small soft brush and dab over the applied decal.