1. Create the images you want to put into the game, making sure they are the suitable dimensions and in PNG format.
2. Gather the files you want to import into the game and place them in a folder on your desktop.
3. Plug in your USB and copy the files to it on your computer.
4. Safely remove your USB (Be safe - not sorry) and then plug it into your PS3. Go to the 'Photo' section of your PS3 menu and your USB should appear there. Press Triange on the USB icon and select 'Display all'. Here you can find the folder you have created with all the files you want to import and press triangle again and select copy. Wait for it to copy and you are done.
5. Boot up your copy of PES 2012 and then go to stadium edit and create your stadium!
Hope this helps!
