Actually the things on the Internet that can be downloaded Like Images are free to use.exept it has ben mentioned in another way ... before getting a Image reading the terms of usage , signing a disclamer before downloading , not being able to get the Images without being registred. this is all what the Internet can offer you if you really wanna protect your creations. Otherwise offering a Image without a disclamer in full size just to download would be a misleading offer to be able in a later state to atack somone who used a fragment of the Product. 

Ilegal it is to pretend to be the originall creator of the Image. you would have to give or mention credit for the usage. actually not the Image is copyrighted but the signature of the Creator.

as a sample : A collage is fully Legal to be created , it is already mentioned into the Description Collage = assemblage of different Images not created by my self but reasemled to present another creation , the new signature of that creator is fully Valid, no matter where the Prints or Images came from and have ben cut out to create the Collage. and is comparable with the creation of Textures, for your models. 

If somone does whant to prevent the usage for further comercial use he is obligated to Watermark the Image and this is what watermaks are here for. or the Image has to be presented as Low resolution or small Print so that it would be actually useless for any other Purposes. 

Every copyrighted Product has to be registred in the copyright office. it has to be set into there achives and you will be given a copyright number , serial witch has to be added into the copyrighted Product disclamer.