What are contrast agents in MRI and how do they work?
1) Contrast agent is used to create a sharp contrast on an otherwise fuzzy image. Consequently, its distribution must have sharp gradient, i.e it must have strong affinity to some type of tissue or physiological liquid. The target may be widely represented in the body, but it still should be a clear target. Or, alternatively, it should be injected into a physiological liquid and have no time to distribute evenly.

2) Wikipedia provides simplistic description. The idea is that to relax efficiently, a magnetic nucleus (including a proton from a hydrogen atom) needs to interact efficiently with chaotic microscopically inhomogeneous magnetic field. Such field may be produces by another microscopic magnetic objects. $\ce{Gd^{+3}}$ has seven unpaired electrons, giving the atom the strongest magnetic field of all single atoms available in reasonable chemical conditions.