Prevent non-image assets from being used as show display_image
Prevent non-image assets from being used as show display_image

Fixes #481

johnathan99j/history-project,newtheatre/history-project,newtheatre/history-project,newtheatre/history-project,johnathan99j/history-project,johnathan99j/history-project,newtheatre/history-project,johnathan99j/history-project,newtheatre/history-project,johnathan99j/history-project