Hi Andrea,
Thanks for reaching out to us!
This isn’t yet possible in our new beta version, however you can achieve this with our program version (which is identical to our previous version and available from ribbet.com/download). In the program, when you click an image you can adjust the Fade and Advanced Blend Mode on the Photo Properties dialog that appears.
If you have any further questions, please let us know!
– Regards, The Ribbet Team