This works for Pickit Business users only.
Your collections will automatically be assigned one of the images they contain as a thumbnail, but for best visual effect, you can upload your own thumbnails that fit your brand guidelines.
To upload a thumbnail, click on the Info button inside a collection. An option to choose a file to upload from your hard drive will appear. Make sure the files you upload match the required file size 500x500px (for your Hero collections, the size is 2000x500px for the web version and 1000x500px for the add-in version).