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