We created a new Python library for Blender called 3DN BIP or 3D Ninjas Blender Image Preview. It allows blazingly fast preview loads in Blender. Furthermore, it can load preview images of arbitrary size. It operates as an
bpy.utils.previews drop-in replacement and does all the heavy lifting for you.
Give it a spin: https://github.com/3dninjas/3dn-bip
We are looking forward to your feedback, proposals and improvements.