For illustrations and SVGs, the first asset uploaded is set as the cover image of the collection. If you want to change this, go to the collection's details page. 

To go the collection's detail page: 

  1. Go to your profile's art page
  2. Click on the pencil for the collection you want to edit
  3. Select edit

The cover image will have a star in the upper right hand corner of the thumbnail. To change the cover image: 

  1. Select the new asset.
  2. Select Actions
  3. Click on Make Collection Cover

via GIPHY