Yes you can add Text to your image by navigating to the Text tab (on the left bar), entering the text you want to add up in the top left box and then clicking Add.
To add a picture as a watermark you can upload it and drag it on from the Photo Basket (at the bottom of the editor).
To export as .png you simply change the Format dropdown to PNG on the Save tab.
Let us know if you have any other questions!
Regards – The Ribbet Team