There are several ways to add images in Sketch. The most straightforward option is to drag an image from another application, like the browser or the Finder, into Sketch, or copy/paste it. This creates a dedicated image layer with two advantages: for one the ratio is locked by default, so you can't mess it up by accident.
Furthermore, you can always reset it to the original dimensions with “Layer > Image > Set to Original Size” from the menu bar. This prevents you from using the image at a larger size than intended.
The second option to use images in Sketch is with a pattern fill. After you have created a new shape, like a rectangle, open its fill properties in the Inspector. There choose the second to last option, “Pattern Fill.” Instead of clicking on "Choose Image..." and selecting the image on your hard drive, you can employ the preview area on the left of the dialog, and proceed similarly as for a dedicated image layer.
Either drag the image from another application to this preview area or copy/paste it there. You can even copy a layer from the layers list or the canvas, to use this exact same clipping as the fill. Now change the fill type to "Fill" in the drop-down below, and you are done.
The advantage of a pattern fill is that you can easily crop it by resizing the shape, without the need to use a mask. However, you neither have control over the alignment of the image (it’s always centered to the shape) nor its original size. Therefore insert the image at the largest possible size, even if it's much bigger than the shape layer.
Image by John Silimann on Unsplash.