You may use images from the web with the following caveats;
1. Does not have a copyright symbol or any such wording indicating copyright or negating free use.
2. The images are to be used in a comparison, for instruction,for personal use, to demonstrate a technical element, to showcase a point.
3. You may not claim them as yours
4. You may not alter/edit them in a public forum;example would be to use parts of the image in a logo, to edit the pixels, unnecessary or selective cropping, especially when dealing with body parts.
5. You may not be critical of the image,author,the image's intentions or purpose other than from a technical standpoint. Example; "the image is underexposed" is OK. "the photographer is a deviant" is not OK.
6. You may not directly profit from the image; example; if you made money when people clicked on the image. The ads on your hubs are not relevant.
7. The images may not be directly taken from the owner's web site. It's OK from Google Images but not from "John Doe's Photo Studio."
For more information on this topic;
US Code Title 17 Section 107