How to render a model for upload?

If you use CATIA the easiest way is Photostudio easy.

Add material to to body.

Answered with a tutorial: https://grabcad.com/tutorials/how-to-render-a-model-for-upload

Firstly, the question mark doesn't mean your model isn't uploaded. Your files can still be downloaded and used by anyone. Its jus that there isn't any visual depiction of your model. . . If you want a picture instead of the question mark you can follow Hailstone's method for doing it inside CATIA or use other third party softwares such as keyshot.
Or you could jus wait for people to come up and render your models. I can assure you, not many models that are render'able are left without renders inside GrabCAD :)

To render ur model simply means to have it in a kind of picture format...maybe JPEG, PNG etc. Follow one of the following solutions:
1. The simplest way to achieve this is to try and save a copy of ur model in a picture format.
2. Try rendering in 3rd party renderers such as photoview 360, keyshot etc ( you can as well check google for any free one you might like). Perhaps your model isnt opening in this renderers, check for the types of format it accepts and convert your model to that format. That should work pretty well.
Once you upload any file in picture format, Grabcad automatically identifies it and makes it a preview of your work....Pls do remember to click "It worked" as a feedback response. Hope its indeed helpful.