Dawn was below 14,000 kilometers from Ceres when this photograph was captured and the resolution is 1.3 kilometers per pixel (in the original image; I zoomed it a bit to display the spots better here).
It’s difficult to explain what these structures are. They may be a group of large spots with loads of small ones, or they may be small bright areas that are crowded together. The bigger ones appear to be worn out around the edges, which means that are still more structures to be seen on smaller dimensions.
We are still uncertain on what they are, either. They appeared blurry in a Hubble picture taken in 2004, as Dawn moved toward Ceres it saw them as two detached spots. They are obviously made out of a material that is more reflective than the area surrounding the surface, and one scientist from the Dawn unit hypothesizes that they could be ice.
Modification, on May 11, 2015 at 19:00 UTC: My pal Joel Parker who is part on the team observing Ceres using Hubble told me that the bright spot seen in the Hubble image is different from the bright spots seen in Dawn images. As a matter of fact, the latest bright spots were not seen by Hubble. Maybe they were not present there(unlikely) or maybe Hubble’s resolution did not see them as bright spots on the surface , contrary to the dark area surrounding them.
I’ll mention one thing: Ceres is entirely dark. It replicates around 10% of the sunlight that strikes it. Vesta, another asteroid visited by Dawn, replicates more than 40% of the sunlight striking it. Vesta’s remarkably vivid, but there are a lot of asteroids which have albedos (reflectivity) on the 20% scale. Some are darker than Ceres; Emily Lakdawalla has great graphic contrasting asteroid albedos at the Planetary Society.
The pictures represented have been overextended in brightness to show the darker surface and brighter spots on Ceres at the same time. Dawn has an apparatus aboard that can record the mineral composition of the asteroid’s surface, and now it has stayed in its science orbit, it will orbit Ceres and beep away from it, making thousands of examinations.
With a bit of luck, we’ll have an answer soon.
I do love mysteries and these spots do count. Even though we will discover what they are made of, there are still more mysteries to come and the next exciting part: Knowing the real purpose of their existence.