You are not allowed to create a "derivative work" (which is what you're doing) without the permission of the copyright holder. Even if you purchase the image, the rights of that purchase may not include the right to create derivative works.
Caveat: What you're using the image for could have a bearing on what you're allowed to do. It's likely that no one is going to throw a fit if you're using it for a school project. It's much more likely that you'll get in trouble if you're using it as the logo for your company. The former is a defensible use under "fair use" doctrine. The latter isn't.
The best rule of thumb is: If you haven't specifically been told you're allowed do it, assume you're not.