Yes, I thought about that since some articles do not actually have good enough pictures inside them, or else they are not accessible by the parser. However, Facebook has the same problem. When sharing an article it does not even let you choose some of the pictures that are actually in the article. This doesn't make it an excuse though. Just because Facebook has a flaw, it does not mean it should be replicated.
I will add the ability to upload your own picture, but only after I see more interest in the feature.