Select the card that you want to add a logo to: 
Select "Edit": 
Scroll down until you have reached the "Logo Square": 
Select the logo square that says "Add Logo Picture": 
You are then given options to access your logo: 
After you have selected your logo select the "Checkmark": 
Depending on your template is what your preview will look like. Then select "Done": 
If you have web links on your card, this may be shown at your preview: 
Shuffle will also ask you this message: 