Why do I sometimes see a downloading 'pie' icon when viewing photostream photos?
That device has only a thumbnail and is indicating the progress fetching a higher resolution image.

From Apple KB - My Photo Stream FAQ


  What resolution are My Photo Stream photos?  
  
  On a Mac or PC, your photos are downloaded and stored in full resolution. On iPhone, iPad, iPod touch, and Apple TV, your photos are delivered in a device-optimized resolution that speeds downloads and saves storage space. Dimensions will vary, but an optimized version of a photo taken by a standard point-and-shoot camera will have a 2048 x 1536 pixel resolution when pushed to your devices. Panoramic photos can be up to 5400 pixels wide.