From Test-Scratch-Wiki
The sprite pane is an area of the graphical user interface of the Scratch program where all sprites present in a project can be easily accessed to modify or inspect. It is a white area located beneath the Stage with the label "Sprites" in its grey header. During small stage layout, the sprite pane has a reduced width, though the sprite icons inside adjust to the new size properly.
Contains
The sprite pane contains a thumbnail of each and every sprite surrounded by a grey box; clicking on this thumbnail will open up its resources, including scripts, costumes, and sounds.When the amount of sprites becomes too great for the size of the sprite pane, the pane becomes vertically scroll-able. There is no limit to the number of sprites that can be present there.
The header of the sprite pane on the left side includes an indication of its purpose, that being a "sprites" label. The right side of the header consists of four buttons used for creating a new sprite from different locations or in various ways. When the stage is minimized to half its size, the "sprites" label disappears due to insufficient room in the sprite pane's header.
Organization
Sprites are organized initially based on order of creation; when a new sprite is created, its icon or thumbnail goes to the end of the sprite pane. However, the icons can easily be dragged around by simply clicking on the icon, moving it with the mouse, and releasing to snap the icon into a new position. All sprite icons are the same size in the pane; a larger sprite does not have a larger icon. Rather, the icon displays the current costume of its sprite in an either compressed or enlarged form to display the thumbnail.
The sprite that is currently selected has a blue box bordering its icon in the sprite pane. This is a useful indication of what sprite's resources and properties are being displayed on the right side of the screen. The box also contains a small "i" in a blue circle in its top-left corner; clicking this allows access to further, more hidden settings of the sprite.
