Press delete to make this part of the image transparent or tap Command+C to copy your selection, making it available in Clipboard. The more you drag the cursor the more of the image will be highlighted in red to show you have selected it. Just click the area you want to select and drag your cursor.
When working with a PDF this becomes a rectangular selection tool. It also provides Lasso and Smart Lasso selection tools, on which more below.
This tool is not available here when working with images.
