Preview Snapping

The icon toggles snapping; you can also do this temporarily by holding Ctrl after starting to transform an image.

Images have multiple snap points; they can snap along their edges, corners, or center.

The drop-down arrow offers the following options:

Snap to

Borders

Snap images to the edges of the render region.

Center

Snap images to the horizontal and vertical center lines of the render region.

Other Strips

Snap images to the snap points of other images.