So matcaps are quite a long time now in blender. Now in 2.7 we have them on all objects with is superb!.. however… from that all stuff one very important option has been forgoten?
Loading own matcap image files will be added later. That needs design and code work to get it stable and memory-friendly.
Probably only because you are using a higher resolution than the ones hard coded in Blender… Conversely, you CAN use a high resolution map to give you much smoother results with some slowdown) than the ones hard coded into Blender!