Quote - what I learned in a course about creating websites was that it is okay to use other people's work as long as you don't take credit for it and it is clearly recognizable who the copyright holder really is

Not even close. International copyright law says you can not use somebody else's images without their permission. Unless somebody who owns the copyrights says (in writing) that you can use it, then you CAN NOT use it legally. > Quote - It is said anything before 1900 is in public domain IF it has had public access.

Nope. Photographs taken more than 75 years ago are in the public domain because they can only be copyrighted for 75 years. Unless they have been rephotographed, in which case the copy has another 75 years to go. Same for photos of art or anything else. Any photograph of the Mona Lisa is copyrighted by the photographer who took the picture. And it does not matter how heavilly you modify an image. If you started with somebody else's image, they own it, not you.