No permissions required
Print to Size does not require any specific permission to access your photos. That is because any photo you add to the page are added directly by you using the iOS photo picker (or other methods such as Copy/Paste and Drag & Drop).
Did you get an error?
Every now and then I receive a query from a user saying that they have started seeing a permission error message when trying to add a photo to the page. Often this happens to users who have not encountered any such issue before when using Print to Size.
The good news is that this appears to be a glitch in iOS. Simply restarting your iPhone or iPad solves the problem.
How to restart your device
It depends on the device and model. Apple provide instructions on how to restart each model of iPhone here and iPad here.
Still Not Working?
Please get in touch to let me know! As for a workaround, the + button is not the only way to add images. You can also use Copy/Paste or drag & drop. See this page for instructions.