Google simply displays results it finds on the web. It could be images from a site that provides images "free to use", but it will also display copyrighted images. You will have to follow the link to the site hosting the image, and determine if it is copyrighted or not.
There is also a feature in the Google Advanced Search that will return only results that are "Creative Commons/Free to use", but you should still double check. "But Google Said So" will not be a valid defense for copyright infringement.
Most of the images there are not free to use. If you want free to use images then the best place to go is creativecommons.org. They will search for creative common licensed images that you can use.
Most people just use the images in Goole/Bing/etc. anyway and most won't get caught, but it's best to make sure not to use images you aren't supposed to use so that you won't have to take them down and replace them later if you do get busted!
One "credible" way to use others' images, and maybe even a beneficial way (to them) is to make the image a link to their site as well, or reference it in a resources section. That might actually be helpful for them in some cases, especially if your site is popular. Unlikely they will make you take that down since it is now a back-link in this case.
No Google images are not free to use and if you use any copyright image then you have to face the copyright laws. If you want free images then there are many sites where you get them totally free and you can use them in your products and ebooks. One site I know is Pixabay, the website contains upto 560,000 thousand free images, but before using an image first see the license at the left side bar. But most of the images there are totally free for commercial use.
No, they are not.
There are not such a term like "Google images" or "Google articles".
If you want to find free to use images for your blog, go to http://openclipart, they are really free, I'm using them for my blogposts.
They are in svg format for doodle video and in png for articles and blog posts.
Hope, it helps.