How to upload only a screenshot ot rendering to GrabCAD?

If you mean add it to someone else's model, click on 'Add a rendered image for this model' below the comment box and choose it from your hard drive, write a something and then click on 'Add comment'. This will work for your own models as well.

If you mean uploading just a render in a separate page, do it like you normally would with a CAD file, just instead choose the rendered image, choose Rendering from the System drop menu and then added it again for a screen shot.

thank W.Wolf

Well It didn't work. I have 2 renders, Kreepster and Blue plastic part that were to go under the render tab and it is under the models tab.
No big deal I guess