How do people make book covers for the Image Manager thing?

You don't need cover-making software specifically, but if you want to make a cover yourself, an image-editing program will work. GIMP is a free image editor and can be used for making covers (link). I have the program myself on Windows (even though I'm not much of an artist), but the link I gave you is to their Mac version.

I'm pretty sure Macs have Paint... I used one myself...

Anyway, you can use any picture or create your own. Programs you can use to make your own digitally are Photoshop, GIMP, Paint Tool SAI, and more. GIMP is the only free one here though. Or you can just traditionally make your own cover by drawing.

I use Photoshop Elements 10 and a free computer app called Seashore, and occasionally I use the smartphone apps PicLab, Instacollage, and Camera+. Hope this helped, love!