When you first export your designs to your project, the first screen will be selected as the project thumbnail for your workspace by default.
You can change a project's thumbnail from the "Dashboard" tab when you are viewing the project. Right-click on a screen and select "Set as Thumbnail" to set the thumbnail for everyone invited to the project:
💁♀️ Zeplin will assign background colors to the project thumbnails — the same color for the projects on the same section — for more visual organization as of Zeplin's Summer Breeze release. On the following releases, you’ll be able to customize them.
☝️ If you delete all the screens in a project, the thumbnail will still remain.