By comparing memory to a painting, the author is metaphorically speaking, saying that memories, just like paintings, they represent things as they feel and not just how they are. Memories can't be the same as the reality. But they can retain most of the things, the feel of it but not in the same sense as the real feel. By saying that "<em>Memory is an abstract painting</em>", the author means that memories holds the best qualities of those incidents or situations just like an artists paints something and that stays as it is represented, not the real truth but in a vague and indistinct way.
