First off, you'll want to do a google search for an image with transparency for whatever it is that you want to do a mockup of. For the love of God, don't make the same mistake that I did and type "transparent onesie" into Google image search. There are a large number of adults out there who wear onesies, so I learned!
Take your rectangle frame tool (on the left side panel, it looks like a square with a cross through it) and make a frame around the entire outside of your image area. Now you can drag and drop any background image onto the ugly old white area!
Right click on the image (a greyish little circle should be visible first), then choose "Fitting" then "Fill Frame Proportionately".
I usually lock the background at this point (right click menu -- Lock), then I don't mess it up as I try to add other things!
Now draw another frame and drag & drop your transparent image into it.
Write some text on the image, or draw another frame and drop a picture... But here's your chance to play with your fonts, colors and designs. Fun!