Recommendations Block: Support more/custom image sizes

Is your feature request related to a problem?

Users currently can choose to display the "Parse.ly thumbnail" or the "original image" in the Recommendations Block.

However, we recently discovered that Parse.ly uses the thumbnailUrl metadata value for storing the image URL, so the "original image" turns out to be the thumbnail WordPress creates. Due to this situation, users needing larger images must tinker with metadata to get the result they want.

Describe the solution you'd like

We could attempt to fix "original image" to be the full original image, but this can also be problematic as full-res images will be large and heavy. Also, although this is essentially a "bug fix", it could also be considered a "breaking change" because of its potential impact.

The ideal scenario would be for the block to auto-choose the correct image based on its current size, or at least give an option so the user can choose the desired image size to be used.

Possibly related: #759

Start with the Recommendations Block and trace how its current “Parse.ly thumbnail” and “original image” choices use the thumbnailUrl metadata value. Review related issue #759 and define whether completion means automatic size selection or a user-facing image-size option, including behavior that avoids unexpectedly heavy full-resolution images.