Hi Kevin - technically - if the thumbnail was applied in Desktop - (depending on the version of Desktop - I believe starting with 2.x) - it should be bundled and included when uploaded to the Qlik Sense Cloud. Sometimes it takes a few minutes for it to update. If that is not the case, then you can manually add the file to the thumbnail.
Check out the attached video (.mp4)
Also see this video as a reference for more information
Please mark the appropriate replies as CORRECT / HELPFUL so our team and other members know that your question(s) has been answered to your satisfaction.
9-6-2016 9-15-53 AM.mp4 3.1 MB