Can I use an image Ive found on the web but doesn belong to me?
It depends on the person or company that created the image–all works created by any person become copyrighted automatically, which means regardless if the creator visually claims copyright ownership, the work is protected with "all rights reserved."
If you don’t any permissions or licensing agreements made clear, you’re required to ask the creator.
Creative Commons License
In some cases, especially with image sharing networks such as Flickr, you’ll find a lot of photographers and artists sharing their work under a Creative Commons (cc) license, which is a license that enables them to grant web publishers permission to use their creative work under a standardized license up front.
So unlike the copyright license, which announces "all rights reserved," the creative commons announces "some rights reserved," usually meaning you’re free to use their work, expand upon it, and share it legally.
Some creative commons licenses may vary depending on the creator of the work, so pay attention to whether they require credit, restrict commercial use, and other points of interest.
