What Dizzie said. Save your background image thumbnail under a different name, then go into the relevant folder, delete the .rsr file for your prop/pose/figure/whatever, and rename the .rsr file for the background image to match the name of the proper file, and delete the background image's prop/figure/pose file. So, for example: Prop.pp2 is your prop, Prop.rsr is the thumbnail Poser made, Proppic.pp2 is the file created by Poser with your rendered image imported as the background image, Proppic.rsr is the imported background's thumbnail. Delete Prop.rsr and Proppic.pp2, and rename Proppic.rsr to Prop.rsr. There you go. Custom thumbnail.