The general rule is that you need permission to use anything that anyone else created -- images, text, video, etc. -- regardless of where you find it. There are narrow exceptions if, for example, you're using the images for commentary (what's known as "fair use") or if the image has a creative commons (or other freeware) license. If you're planning to go into the world of online media, copyright infringement is a huge (and costly) issue to run afoul of. Align yourself with an intellectual property attorney -- even if it's to get the basic consultations -- so that you can steer clear of the pitfalls.