Sketch contains a couple of the most common bitmap editing capabilities to help prevent you from jumping between different design tools.
To start editing an image, select it on the canvas and double-click. You will see the Inspector update with two disabled options. In order to use these you first have to select an area on the Canvas by dragging the cursor to create a rectangular selection area on the image.
Note that you can hold the Shift key to add to an already-existing selection, and use the Option key to subtract from it. Once you’ve made a selection you can either cut/copy the selection and use it for a new bitmap layer or select one of the two options in the Inspector.
- Invert: The area that was not selected will be selected now and vice versa.
- Crop: Crops the layer to only include the selected area.
Once you are done editing, click anywhere outside the image, or press Return or Escape to exit.