Thanks for contacting us! Using the Constraints drop down as you’ve described is the correct way to have your images perfectly fit the paper size selected. However what it sounds like is happening in this case is that the Printer settings are what’s causing the issue. If you choose 8×10 the image will be exactly sized to match 8×10 proportions. So just before printing you will need to go into Print Properties and have a look at what options are available for your printer. It will most likely be a case of changing the option “Print to Fit” or similar, which will tell the printer to fit the image to the paper.
Another thing you can try if this doesn’t do the trick is printing from Ribbet’s Save > Print tab.
If you still have an issue, could you please email us an example photo that you’re trying to print so we can check it looks correct (to support [at] ribbet.com)? Thanks!