This works for Pickit Business users only.
Your collections will automatically be assigned one of the images they contain as a thumbnail, but for the 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 matches the required file size; 500x500px for the featured collections and 1000x500px for the hero collections.