The weirdest thing is that I've done it before! Twice! and both times it worked perfectly in IMVU.

I made a 512x512 flat plane poster mesh and a 400x600 one. A couple of months ago.
Now I'm trying to make a 600x400 flat plane poster mesh. It works it Sketchup, but importing it into IMVU produces the most atrocious looking thing you can imagine... When a picture is added to it, the picture is repeated over and over and over again.

I save it. Go to Meshout, Export the IMVU fileset, and poof. I get a pig. Pigs are great, but I'm trying to get a poster mesh.