I have a separate (from the rest of the bathroom) toilet room in the master bath. I've got a framed print of Picasso's Don Qixote on one wall and a framed print of a Matisse nude on the other wall (my tastes have changed to where I'm not going to put prints in common areas and I spent too much on the framing to toss them). There's a niche next to the linen cabinet where I have a standing 5 or 6 shelf rack of rolled bathtowels, handtowels, and washcoths arranged descendingly from lightest to darkest, top to bottom. The tub is open on one side, with a windowed wall opposite and the shower on the bow end. On the stern wall above the towel rack I have a framed abstract oil that I won at a charity auction.
I've got framed posters of my favorite Broadway shows that I've attended in my second bedroom bathroom, and a framed piece of artwork in the downstairs/guest bathroom. However, in the 5 years I've lived there the showers in those bathrooms have been used never and twice, respectively.
I'd suggest towels to deaden the echoes. Plants would work also. I haven't had any problems with my master bath artwork, but that's probably due in part to the fact that that the prints are in a fairly self-contained area and (moreso) because I take 98% of my showers at the gym.