Media selections
You can make media selections in a variety of ways. The most obvious is simple point and click. If you want to add more media items to the current selection, then press Command (⌘) while clicking on additional media items and they will also be added to the current selection.
You can also make selections by shift-dragging out a selection rectangle to intersect those media items you wish to add to the current selection.
Finally, you can select everything on the media page by pressing the shortcut key Command (⌘)-A and all media items will be added to the current selection. Invert the current selection by pressing Shift (⇧)-Command (⌘)-A.
Keep in mind that in Storme all actions that can operate on a single media item can also operate on a selection of media items.