I've spoken with at least a dozen artists who put the position on their resume, all of whom have "painted mattes". They have all meant the frame by frame filling in drops in keys for composite shots. The painting of the actual matte.
In the olden days the matte painter was the guy who created the image that filled in the hole they cut out.
Watching "North by Northwest" last night, the terrific mattes stood out -especially the United Nations.
These were painted by Matthew Yuricich .
In this little interview he expresses a client process that's different from the typical one.
Sometimes we'll do it his way, but usually find its better to get the client involved early.