You can do it via your computer's iTunes. If you are on iTunes 11 on your computer then you may want to re-enable the left-hand sidebar via View > Show Sidebar (option-command-S on a Mac, control-S on a PC), which might make it easier to navigate and my instructions may make more sense.
To sync photos, connect and select your iPad on the left-hand side of your computer's iTunes (if you've enabled the sidebar), 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.
If you haven't enabled the sidebar, then from your library click 'iPad' at the top right of the screen and you should get a series of buttons along the top of the screen, including one for Photos
Thank you for the tip. I'll try this tomorrow after I complete some preparation for a class I'm teaching.
A question: What is I-Tunes 11?
I downloaded I-Tunes while working with one of the able Apple support persons who work with novices like me who've only worked with Windows desktop PCs (for the past 30 or so years) and the Mini is a wholly new adventure.
By the way, because I infer from your answer that images from the hard drive on my PC may be cut rather than just copied in the suggested process, I'll copy my initial images into a new folder on my PC's so as to leave the original images intact. I assume the Mini cannot read or convert Nikon Raw files, I'll convert this group with Lightroom as I save them to the new image folder.
iTunes 11 is the current version of iTunes for computers, it's got a different layout to previous versions and by default hides the left-hand sidebar that always showed on previous versions.
The syncing of photos should work with raw photos, though they will be converted as part of the sync process - the original versions on your PC won't be changed, iTunes will create separate versions to copy over.
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; DSCN0164.jpg).
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. (You can not export using the camera connection kit.)
Secrets of the iPad Camera Connection Kit
In Safari on your iPad there is a link to the user manual for your device. It is a good idea to look that over and especially learn about syncing your iPad to your computer. This is where you use iTunes on your computer. You download iTunes from Apple for free. Then you connect your iPad. Once you select your iPad in iTunes on your computer you can select folders of photos that will be copied to your iPad when you click on sync. Same goes for music. You can also use iTunes to copy over other files to use on your iPad. The sync process is an important part of using the iPad.