Changed the minimum size in which an image is cached.

Presently when caching images they are only saved when their file size is greater than 1KB. I'm not sure if there's a technical reason for this, but as best as I can tell the smallest, valid image size is 34 bytes (1-pixel GIF) based on [this StackOverflow post](http://stackoverflow.com/questions/2570633/smallest-filesize-for-transparent-single-pixel-image#15960901).

This commit changes the minimum size so files greater than 33 bytes are cached. It helps ensure that SSL is maintained (and also limits analytics tracking from 1x1 pixel GIFs used in feed articles).