Pretty much the only legal implication when using/editing graphic images is copyright, which generally means "the right to copy". Alot of images are either copied from someone elses design or directly copied then edited, this is not neccessary against any copyright laws because the image is not the same as the original. Although aslong as the credit is given to the original creator it is possible to actually copy and use the image. In order to actually copy and resuse someone elses images you need to ask the permission of the owner.
If the image has a logo or stamp on or near the image it signals the ownership of the image and who has the rights to that image, if a logo isn't on or near the image then the rights of the image might be owned by the owner of th website, so to use the image in anyway contacting the website owner may be the best option.
Any image that is copyright free can freely be used, edit and distributed to anyone you desire, these images are usually images that are free with graphic packages or websites that clearly identifies their images as copyright free.