For Firmware Versions < 2.80
You need to connect your PSP to your PC using a USB cable and activate the USB mode on your PSP (found under settings, second option from the top). Once connected, look inside the X:/PSP/PHOTOS folder (where X is your PSP Drive). Then all you have to do is drag and drop the photos into the folder!
Once you exit the USB mode on your PSP, access the PHOTO menu from the PSP menu and you should see your photos there. Best size is 480w x x272h.
For Firmware Versions 2.80+
You need to connect your PSP to your PC using a USB cable and activate the USB mode on your PSP (found under settings, second option from the top). Once connected, look inside the X:/PICTURE folder (where X is your PSP Drive). Then all you have to do is drag and drop the photos into the folder!
Once you exit the USB mode on your PSP, access the PHOTO menu from the PSP menu and you should see your photos there. Best size is 480w x x272h.