Putting a 301 redirect on the path will do the trick, it will ensure that any references to the old path are redirected, as well as updating search engine indexes.
Hijackers will be able to find the images whatever path they use, they just need to look at the source code or right-click on the image to get it.
b) Have noticed a lot of sites using the complete absolute path for images. Why is this? Does this create some sort of assurance that the images are being served from the same site/source?
It's probably more to do with people not understanding the way the internet works, or CMSs that aren't sophisticated enough. In the old days, it might have been useful when people often downloaded pages for later reference, and that would ensure the images remained in place. With broadband now commonplace, very few people download pages these days, so it isn't an issue.