First, you should remove the pictures currently in the Camera Roll (Photos), so you won't be duplicating everything. You can do this by connecting the iPad to your computer and then going into iTunes, and under "Devices" on the left hand side of the screen, click on your iPad name. A window will open up with tabs on the top. Select "Photos", under "Photos", un-check "Sync Photos from..." , and sync, this will delete all of the photos on the iPad
You need to setup a folder, on your PC under "My Pictures", such as "iPad Pictures" and make all of the folders you want, with the pictures you want in them (copy, move or drag and drop, your choice), under that folder. Then go into iTunes, under PHOTOS and select "Sync Photos from" and change "My Pictures" to "Choose Folder" and then choose the folder you created "iPad Pictures" open it and select the folders you want to sync to the iPad. Then sync them.
Remember items that are checked will be put on the iPad, items that aren't checked will be deleted if they are on the iPad already. If an item is already on the iPad, iTunes will just skip it when syncing and go onto the next item.
