How does one make proper rendered thumbnails?

I've noticed that whenever I submit a model, I tend to get the thumbnail changed. Am I doing something wrong?

For N64 models, I usually just render the model in kHED, because I feel like the primitive form of shading the software renders behaves more like how N64 games used shading by vertex, rather than thru dynamic shaders.

My only guess is that they don't have the background they use for model rips?


Here was the full-sized render of Rusty Bucket Bay (I know there was an issue with stretching, dunno why that happened)



Here's the one The Models Resource used (I'll admit, I like this one more because it shows the whole map)