Lovely tiles! I like that they cover the back wall. The sink looks great too! Here are some guidelines to think about for a wallpaper:

• Your tiles are a small scale. So pick a wallpaper that has a medium or large pattern to contrast with the tiles.
• I would stay away from geometrics, again for contrast. Something with a flowing, natural feel would work well - think florals, branches, or grasses. If your bathroom is headed in a more modern direction, something with abstract swirls possibly.
• In order to let the tiles stand out, use a wallpaper that’s tonal - as in subtle color variation and very low contrast. You have a lot of leeway in what color you select. Personally, I would use something in varying neutral shades, like cream to taupe. But you could us a paper with a subtle pattern in shades of greens, blues, or reds. Maybe dark grays or near black if you want to get dramatic.

Hope that helps! Best of luck.

Replace that mirror with a back lite light and remove the existing light before adding wallpaper.