"What is a way of decorating the tub while avoiding mess ?"
The answer is directly related to how the tub is exploited. If you concerned with the appearance than its function, so you have to keep around the tub some items that not only look lovely but will enhance its use.
Think about flowers or even include candles if you have much space, bath oils, and soaps, as these are pleasing to both the eye and the bath user.
Pictures are definitely not advised as they could easily get destroyed by a soaking, or the dampness that results from the tub. The humidity and natural light can encourage plants to prosper and they add a soft touch of nature to the bath. The point is to make it romantic and inviting as well as functional.