I have the exactly same problem.
I have a lot of (21 MB) animations in my project and there seams to be some magic limit.
Its now impossible for me to import more sprites or more animations than this limit. I tryed to delete other sprites in replace for new animations, which worked. But i can't get anymore than this magic limit loaded. If i try, first the opening dialog is very unresponsive and if when i manage to select and load images Construct freezes.
Windows7 - 8GB RAM - Nvidia GTX 560 Ti - Construct2 79.4 64-bit