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. The Inspector will update to show two selection tools. First you have to select an area on the Canvas, and then pick the tool you want:
Selection: Select a rectangular area on the image.
Magic Wand: Click and drag anywhere on the image to select an area. The further you drag away from the start the greater the tolerance will be.
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.