Easy-PhotoPrint doesn't store your photos anywhere. All it does it display all of the images which are stored in a particular folder on your PC. (e.g.'My Pictures').
If you're certain that you never want to see a photograph again, and you're happy to delete it completely from your PC, simply go to the relevant folder (e.g. 'My Pictures'), right-click on the image and select 'Delete'
If you want to keep the image on your PC, but not have it display when you open Easy-PhotoPrint, you need to move it to somewhere else. Create a new folder, somewhere on your hard drive, called something like 'Spare pictures'. Then go to the folder where your pictures are currently stored (e.g. 'My pictures'). Right-click on an image you want to move and select 'Cut'. Then go to your new folder. Right-click within it and select 'Paste'.
If you want to delete or move lots of pictures remember that you can highlight multiple pictures, at the same time, by holding down the Ctrl key while clicking on each of the images you require in turn. Then right-click on any of the selected images and proceed as above.
Chris