Use thumb images when available on selecting cool pins

In the admin, when selecting cool pins for a category, show the images from the thumbnail image when this is available
On registration user thumbs to show pins

When registering and selecting the initial 5+ pins, show the thumb of the
image whenever possible
Retain the last category for pin loaders page
Merge pull request #18 from camilotorresf/register-pin-selection

Optimization to pin loaders page