Hey Ben!
On your PS2 you should have a couple of USB ports. Many USB flash memory sticks are compatible with the PS2, and GT4 has the coding built in to recognise and use the USB memory sticks in teh same way your |computer would do. Plug one into the USB port and if it's like mine, it'll have a little power LED to show that its connected.
When you go to photo mode and take a picture, you can choose to save it to the memory card, or USB memory. The way I do it is to save to memory card, then go to your garage, and choose Photo Lab. In there choose photos, and select the pictures you want to put on the USB memory device. In the right hand column one of the options is to "transfer to USB memory", choose that and your images will be transferred onto the USB memory device plugged into your PS2
GT4 will create a folder on your memory stick called 'GT4' and put the images in there. Take the USB memory stick out of the PS2, plug it into your PC and transfer the pictures onto the computer, and then Robert is very much your mother's brother!
I use
www.imageshack.us to host my screenies, it's nice and easy to use. Oh yeah one last thing - the GT4 photos are in a resolution of 1280*960 as standard, good for your desktop wallpapers, bad for the forum and 56k users, ha ha! I resize all mine to 640*480 so they're a little easier to deal with
Hope this helps!