Most artists will tell you somewhere on their web pages what the terms of usage are, so read carefully . To check on authoring information within an animation, download an animation program, such as Ulead's Gif Animator, open the animation and read the comment file. If there is no information available, it doesn't necessarily mean the animation is shareware - because this info can be removed by optimization programs. Most often you will find an artist's name who you can contact for copyright and credit information. Changing a graphic without permission.
Many people think that by modifying the original they are safe.
Coca Cola and other household names have proven this to be incorrect. It is an infringement of the copyright. If you want to modify, then seek the artists permission. Do not link directly to a graphic. This can cost the artist a lot of money which could result in their web site being shut down.