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.
If your model is not showing the correct thumbnail image, it is often the result of the model not being done processing. If you've waited for a bit of time and it's still not showing properly, you can try updating the thumbnail image again by clicking Save View.