Hi, I want to use Sketch to import UI icons and backgrounds such as you would find on sites like Unsplash.com.

You can make a mask for the image - just right click on the image and select "Mask". This will make a box that the image is bound to that you can resize as needed. It will not scale the image, it's more of a crop.

There's 3 ways you can do that.

1. Like converting the object in which you cant to add the image into a mask MaskRebecca said,

2. Create the shape where you want to add your image, i.e a square, then on the right menu, on the "Fills" category, click on the color selector. It will open a small box to pick up a color, and at the top of that box there will be 6 squared grey icons. Click on the 5th one. It will then add a pattern to your shape by default. Now, click on "Choose Image", select the image that you want from your computer. Then below the "Choose image" button, click the dropdown and select "Fill", it will fit your image into the shape container maintaining the aspect ratio. You can have a look at this screenshot I took: https://www.dropbox.com/s/s1l5n96k037m9kx/Screen%20Shot%202017-05-18%20at%2020.33.12.png?dl=0

3. Download the Craft plugin from InVision -it is free-, and use the Data feature to add any image or text from the web to your designs, like going to Unsplash and add an image with just one click . https://www.invisionapp.com/craft#data I honestly find this plugin impressive, the functions Data and Duplicate are specially handy. :)