Step 1) Click on the element you want to round the corners on (photo or text box) and in the top Frame menu, select Rounded Corners

Step 2) The corners on your element will now be rounded and you can adjust them by selecting Rounded Corner Settings in the Frame menu

Step 3) In the Rounded Corners menu, you can adjust the corner radius by using the slider or typing in a number. Select Done when you are happy with your edits. 

*You can click on the menu and drag it to a new location on the page if it is covering your elements