How do I import photos from Mac into Keynote on iPad?

By far the easiest way of doing this is via iCloud.

You have a Photos app on both devices. Configure both the Mac Mini and the iPad to sync photos via iCloud.

Now (assuming you have enough iCloud space available in your account) your iPad will have full access to all the phones on your Mac mini (don't worry, they don't all download, only the ones you specifically select, so it won't take much space on your iPad)

Right - I assumed that. It sounded like you fist needed to get the photos onto your iPad, for which iCloud Photos is key.

To actually add a photo, just go to your slide and click the + icon to add content. Choose Photo or Video and Keynote will dig into your Photos Library to let to pick an image to add to your slide.