To sync photos from the your computer to the iPad's Photos app you need to use your computer's iTunes (unless you have the camera kit for copying from an SD card or direct from a camera).
To sync photos, connect and select your iPad on the left-hand side of your computer's iTunes (if you are on iTunes 11 then you can enable the left-hand sidebar via View > Show Sidebar, control-S on a PC), and on the right-hand side there should be a series of tabs, one of which should be Photos - if you select that tab you can then select which photo folders to sync to the iPad. There is a bit more info on this page. You will need to sync all the photos that you want on the iPad together in one go as only the most recent photo sync remains on the iPad - synced photos can't be deleted directly on the iPad, instead they are deleted by not including them in the next photo sync (is that what you mean by camera roll ?).
Deleting photos from iOS device
iOS and iPod: Syncing photos using iTunes
iPad Tip: How to Delete Photos from Your iPad in the Photos App
Another Way to Quickly Delete Photos from Your iPad (Mac Only)
How to Delete Photos from iPad
How to: Batch Delete Photos on the iPad
(With iOS 5.1, use 2 fingers)
How to Delete Photos from iCloud’s Photo Stream