The paper Geometry and spatial distribution of lenticulae on Europa gives a good description what is going on.
As I mention in my comment the bumps/domes are called lenticulae, Latin for freckles. In addition to the gray/blue ones in the photo there are also red ones elsewhere on Europa.
Lenticulae tend be "roughly elliptical" in shape and can occur as either domes or pits, or both - see the picture above. General observation of lenticulae are:
(1) pits and domes have similar sizes; (2) chaos are larger than the other lenticulae; (3) pits are clustered within the trailing antijovian quadrant and the leading subjovian quadrant whereas domes, dome/chaos, and chaos terrains are more uniformly distributed; (4) the areal density for all lenticulae is not uniform; (5) lenticulae do not divert the path of younger lineaments such as ridges.
The conceptual model for the lenticulae is that they are,
... a surface expression of dynamics within the ice shell at a different stage of lenticulae evolution. The similar size and shape of pits and domes suggests that one may evolve into the other. Because domes are more numerous and more uniformly distributed than pits, they are more likely to represent the end stage of this evolution, assuming the end-stage leaves a longer-lasting surface expression.
Other observations are:
- That lineaments do do not offset pits
- Pits have a preferred northwest-southeast elongation
- Pits may be an earlier stage in the evolution of lenticulae
The lateral dimensions of the lenticulae range between a few kilometers to tens of kilometers. The heights of the domes typically vary from 40 to 100 m. The pit bottoms are up to 200 m to 300 m deep.
Proposed processes for the formation of lenticulae are plumes and convection in the ocean and plumes and convection within the ice shell, melt throughs, or cryovolcanism.