Q:

How can I alter a square image into a circular one in Sketch?

I want to take a rectangular image in Sketch and give it a circle/radius style.  
For example: 

A:

That's actually quite easy. There are two ways to do it:
1. Use a Mask
Best way IMHO, as it gives you more flexibility / editing options.

Import your image:

Draw a circle that will mask your image:

Move the circle behind the bitmap using the Backward toolbar icon:

Use the circle to mask the bitmap, by selecting it and choosing Layer › Use as Mask from the menu:

Ta-da!

As an extra step, you can group your bitmap and the circle mask, to stop the circle from masking other shapes:

To add a border: add another circle shape placed above the masked image. 

2. Use a pattern fill

Draw the circle where you want your image to appear:

Select Fill › Pattern Fill in the inspector:

Click Choose Image… and select your bitmap:

Set the fill options to Fill in the drop down:

Add a border to the circle as you normally would.
Ta-da!

Hope it helps!

