If you want to upload an image overlay using that online tool, you have to upload it as a 3D model, and to do that, you need to have a model in the MD2 format. (You can generate one of these by creating the model in Blender 2.49 and choosing MD2 as your export option. If you have a version of Blender that doesn’t support this type of export, Junaio provide a downloadable script on this page.)

So, in addition to the replacement image, you’ll need an MD2 “screen” to display it on. You can make one of these by going into Blender and creating a 2D plane with the same aspect ratio as your image. You then upload that as the requested MD2 Model and use the replacement image as the Model Texture. (You may then need to play around with the Scale option in the Junaio GLUE Channel Creator to get a neat overlap, or go back to Blender to resize.)

Apologies for misleading people. Hopefully, this doesn’t turn the do-it-yourself virtual billboard defacing into too arduous or complicated an endeavour… I personally still think it’s still worth trying!