For photographs, you need permission to use the photo unless it's a creative commons image. For characters, there are usually trademark issues, but it depends on what it is. You need to find out what the licensing and trademark situations are with the images you plan to use, because each one will be different.

For any image/photo you want to use, you must own the copyright or have the permission from the copyright owner to use the image/photo.

Also, read recent post: Getting sued for photos on blog?