Once you've uploaded your model to Sketchfab, a thumbnail image will be processed and automatically generated. You can see this thumbnail from a number of locations on your Sketchfab website, including the Models tab.
If you'd like to change the thumbnail, you can click to open the model page, move the model to the position and angle you'd like, and then click Save View in the top-left corner of the viewer or inside the 3D Settings.
It's possible the correct thumbnail image for your model might not show right away. It could take a few minutes for the thumbnail to appear because our servers have to process the model in the background and generate multiple resolutions.
If you've waited for a bit and the thumbnail is still not showing properly, you can try updating the thumbnail image again by clicking Save View.