By default the cover of your publication will be: the first page of the PDF, the first slide of your .dink file or the first image of your video. It is however possible to change the covers of your publications.
Go to the dropdown menu of your publication and choose 'Icon upload':
And upload your image via drag & drop in the upload field, or by clicking in this field and then selecting the relevant image file:
Click 'Save' and after refreshing your app, the new cover will be visible for this publication.