This seems more hoax than myth, but either way, it’s one that has been going on for almost a hundred years. The New York Times carried this story a few days ago.
The painting was purported to be one Mrs. Lincoln had made of herself as a surprise gift for the president, but before she could give it to him, he was assassinated. Turns out it’s another woman’s likeness, and the painting was retouched by a vaudeville actor, no less, in the 1920s and sold to Lincoln descendants along with the heart-wrenching story. The forger changed the hairdo to look more like Mrs. Lincoln’s and added a brooch that showed a picture of Abraham Lincoln. The scam earned him a couple hundred bucks. Art conservators who cleaned the painting discovered the over-painting and realized it wasn’t Mrs. Lincoln at all. For the whole story, see