Adding a custom thumbnail image for your Library can make it easier to gauge its contents at a glance.
Creating a custom Library thumbnail
To create a custom thumbnail, first create a square (1:1) Artboard anywhere in your Library. Then, create your thumbnail design, right-click anywhere on the Artboard and select Set as Thumbnail.
If you haven’t set a custom thumbnail within your Library, Sketch will use the first Artboard on the first page of the document instead (provided the Artboard is 250x250 px or larger).
You can also set a custom thumbnail for regular documents.