Where?
Go to the 'my upoalds' page
Just underneath the title there is a line with subtitles:
Details | Images | Prints | Lightboxes | Designs | Extended Licenses
Choose 'prints'
At the right bottom, 'select all'
Don't forget to chechmark the bottom left box to that it becomes "You are currently allowing prints ", otherwise the new setting won't work
And then select the 'all on' underneath
Kind of a tricky setting, it would be much easier to have it set along with the Extended Licenses option.