Adding a custom thumbnail image for your Library can make it easier to understand its contents at a glance. You can also set a custom thumbnail for regular documents.
To create a custom thumbnail, create a square (1:1) Artboard anywhere in your Library, 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 (providing that Artboard is 250x250 or larger).