This is only possible with iPods with a click-wheel. You can put your iPod into Disk Mode through iTunes or manually.

iTunes - Plug your iPod into your computer. Select the iPod from the Devices menu. in the Summary tab, click "Enable disk use".
Manually - Press and hold the Menu and Select buttons for at least 6 seconds. Wait until the Apple logo appears. As soon as the logo appears, release the buttons and then press and hold Select and Play. Hold these buttons until the Disk Mode screen appears.
 If you put your iPod into Disk Mode manually, connect it to your computer after Disk Mode has been enabled., If you are using Windows, the iPod will appear as a drive in the Computer/My Computer/This PC window (⊞ Win+E). If you are using a Mac, the iPod will appear as a drive as a desktop., Open the "Photos" folder on the iPod. Drag and drop or copy the photos you want into the folder., Once the photos are finished transferring, eject the iPod so that you can disconnect it safely. In Windows, right-click on the iPod drive and select Eject. If you are using OS X, drag the drive to the Trash.
Put your iPod into Disk Mode.
Connect your iPod to your computer.
Open the iPod on your computer.
Copy the photos onto the iPod.
Eject the iPod.
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