Photos that you sync from a computer can't be deleted directly on your iPad, instead they are removed by not including them in the next photo sync - so you need to select and sync all the photos that you want to copy from your computer together in one go.
To sync photos, connect and select your iPad on the left-hand sidebar of your computer's iTunes (you can enable the sidebar via control-S on a PC), and on the right-hand side of the iTunes screen select the Photos tab and select all the folders that you want to sync.
If your photos are under different parent folders on your computer then there are third-party photo management apps such as Photo Manager Pro which give you more control over copying photos from your computer (and which allow you to delete photos directly in the app).