To quickly remove the background behind an object or logo (say, to place the object over a new background), activate Preview’s Instant Alpha tool by clicking its icon in the toolbar (it looks like a magic wand). Next, click and drag atop the background; if you don’t see the Instant Alpha tool, click the Markup Toolbar icon to display all the markup tools (it looks like a toolbox). As you click and drag atop the background, your selection appears as a pink overlay that indicates the areas that will be removed.