As Gnaster729 said, you only can use free pics from Internet when they are explicitly licensed to do so.
The commercial nature of your usage only matters when the license doesn't allows commercial usage. For example, if you find images licensed under Creative Commons BY-NC, you can't use those images in a pdf you sell, although you could use them in a pdf you distribute for free (at least if it is not an advertisement). For other uses, beware that the limit between commercial and non-commercial uses is quite fuzzy.
There are other free licenses that allow you commercial use, but you should abide to license conditions and they may include not modifying the image, properly attributing it, licensing your work or at least your copy of the image under the same free license, and so.
Furthermore, you should do some research to be reasonably sure that the freely licensed image you use is not a copyright infringement in itself - for example, somebody could have grabbed a non-free image from the Internet and uploaded it to Flickr under a free license. Even sites like Wikimedia Commons which actively search and delete copyrighted content (often to paranoic levels) aren't free of some copyright infringements.