A lot of times I will answer a question, OP will post "Thanks very much!" And then leave, not accepting the answer that fixed their problem. I know I can just give them a gentle reminder (asking them to consider accepting), and on most occasions I do and they do end up accepting, but the site should be intuitive enough for them to do this by themselves. So when they post "Thank you" or "Thanks" the site could display a tool tip saying to "accept the answer if it helped best".
The message on the image could be changed to be less forceful I guess.