Connect your iPad to your computer and select it on the left-hand side of your computer's iTunes, select the Photos tab on the right-hand side and select which photos to sync to the iPad. You 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 - not including them in a subsequent sync is how you then delete them from the iPad.
More info on syncing photos here : http://support.apple.com/kb/HT4236
Another way. You can use a USB flash drive & the camera connection kit.
Plug the USB flash drive into your computer & create a new folder titled DCIM. Then put your movie/photo files into the folder. The files must have a filename with exactly 8 characters long (no spaces) plus the file extension (i.e., my-movie.mov).
Now plug the flash drive into the iPad using the camera connection kit. Open the Photos app, the movie/photo files should appear & you can import.