Get on the internet, do a search for Old Campers (RV's). Copy the image you like to a Word Processing application and print the image. Take the image and cut it out of the page, trace it on the top of the board, and then paint it with a set of modeler's paints, (like Testors). If you personalize it in such a way as this, it add so much more character and indivudualizes your work. You can gloat because you did the paint and didn't just paste something ready made on it.
If you need the image bigger, you can expand the image over several pages by zooming in with the word processor and print smaller sections blown up and piece them together. Or do the proportional tracing by using a square grid line and free hand the outline. Either way, it will be uniquely yours.
Beside, if the corn bags scar the paint, you can touch it up easily. If you purchase a ready made graphic, and it get's scared up by a bag sliding over it or pounding on top of it, it will rip and tear and wear and look bad after a while. You won't be able to fix it, and removing it will next to impossible.
Just some suggestions.
Good luck.