How do I cover up an unwanted image on a mug? (see details)

I received a mug the other day as a promotional item at a fair. The mug has an image (not a corporate logo, but something else that I would rather not go into detail here) that I do not like. The mug, however, seems nice and sturdy, and I don’t want it to go to waste.

My idea would be to simply cover up the nasty image (which is not very big). Since the image is on a mug, whatever I use to cover the image up would have to be waterproof. Also, I plan on taking the mug to public, so simply scratching out the image with a Sharpie does not seem like a desirable option to me.

I was thinking of using waterproof stickers? Would a waterproof sticker work here? If so, does anyone know where I can get waterproof stickers at a reasonable price?

Any additional information you could provide would be welcome.