To my knowledge, ArtStation and Tumblr are the two common sources which previously had better quality images available but have removed such access. I suggest for the legacy posts added to the site before access was cutoff, we create a tag to indicate this, and document what exactly was available for new users who may not know where larger images are from that don't match their source (A user asked me about this in post #2744665). The defunct original image situations are described in part at artstation sample and tumblr sample
To go about this, I suggest orphaned_original
or some other name if more popular (defunct, legacy?). We could either just have that umbrella tag or, as I suggest, in addition to this tag have site-specific tags such as orphaned_artstation_original
and orphaned_tumblr_original
and more as we come across them.