Painting to collage, painting to mosaic, photo to mosaic.
"Copyright" regards the "expression of an idea." Someone expressing an idea is the author of a work - e.g., a painting, a book, a photograph, a mosaic - and has the right to keep others from copying that work. In other words, only the author has the right to copy the work; i.e., the copy...right.
While you and I can both stand in front of Mt. Rushmore and paint, draw, or photograph the U.S. Presidents, neither of us would be infringing each other's copyright because we are expressing in our individual way the "idea" of Mt. Rushmore.
It gets a little hairier if I copy your expression of an idea. This bleeds into an area of copyright law known as a "derivative work." That is, if I'm standing behind you painting your charcoal expression of Mt. Rushmore, in a sense, I'm deriving my work from yours.
A copyright attorney would have to understand what the underlying subject (idea) is and whether you are creating your work directly from a well known subject (like Mt. Rushmore) or whether an author had created something unique from which you are deriving your work.
You can find some basic copyright information here: www.copyright.com
Yes, there is an issue: Such copying likely constitutes copyright infringement (unless the original work was in the public domain). For a real-world example similar to what you have asked, please see the discussion of Shpeard Fairey's Obama HOPE poster copied from an AP photographer's photo at the first link below.
For an overview of copyright generally, please see the post at the second link below.
Disclaimer: This post does not constitute legal advice and does not establish an attorney-client relationship.
You should be aware that this very question has been answered many a time by many a court in many a different way. However, that being said, as a general principle, you cannot avoid infringing a copyright simply by changing medium. This is very well illustrated in a famous case Rogers v. Koons. There, celebrity artist Jeff Koons took a photo of puppies authored and copyrighted by someone else and made a sculupture depicting the scence within the photo. He was held to have infringed the original copyright by creating an unauthorized derivative work (the right to create a derivative work is one of the exlusive rights of the original copyright holder). So you should be wary of how and what you intend to reproduce in another medium.
I hope this helps.
Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.