Just mount your Droid to your PC using the USB cable*, then create as many folders on your SD card as you want.
The Gallery will find them. Same thing goes for folders like "Music", "Ringtones", "Wallpaper", etc...
*- Turn on your Droid and connect it to your PC using the USB cable
- When you hear the "Drooooiid" notification, you'll see a USB icon in your status bar (above)
- Pull it down and click on "USB Connected"
- In the resulting dialog click "Mount"
- It should now show up as just another disk drive in "My Computer" on your PC