Thanks for the votes!

Take a white circle with a black background (which can be constructed on Picsart, or oh can search for it on safari)

Take your image, and size it over the white circle. Click darken.

Add your fonts.

If your background is any colour but black, you can stop here and just click lighten or screen.

IF YOUR BACKGROUND FOR YOUR STICKER IS BLACK, CONTINUE WITH THE TUTORIAL.

Click tools,  and click shape crop. Click the circle shape and adjust it to the correct size. It's now a png.