Changing the thumbnail image in ArcGIS Online Attachment Viewer

[MelanieWilliams1]

For our internal and external use, the Attachment Viewer template has become a big hit for applications focused around accessing pdf documents through a mapping interface.  One request that comes up every time though, is if the thumbnails for the attachments (in a map focused view) can be changed to something else other than the pdf icon.  

Would be great if we could set the thumbnail as the label or the map symbology with the label.  There may be a way to do this but I have not come across it.  Am I missing something?  

This has been a really helpful template in several cases for us!

Thanks!