What makes a successful image?
That's like asking "How do i succeed in life" or "How should i make money"? 
A topic worth several books.

The simple answer is that an image is successful if it involves viewers deeply on all their levels; intellect, emotion, spirit and physical perceptions.

No one has every agreed, and probably no one ever will agree on "What is art?", which is the question your question begs. Too much depends on what the viewer brings to the viewing experience for there to be any standard answer that works for everyone every time. Happily, we are all unique.

What makes a successful image maker is another good question. I'd say that is someone who successfully taps into their unique individual nature to relate their inner things to the outer world and expresses that in strong and original images. Such work can turn a few million colored dots into something with a bit of a soul.  Something that can be successfully transmitted to others visually.

There is a forum here at Renderosity on art and creativity and such.

There are a lot of "rules of thumb" and lots of principles and methods in image making (see my book) and visual communication, but all rules are regurally broken successfully by artists - so they are really only guidlines.

Learning to see keenly and be an excellent observer helps a lot - and i recommend digital photography (with instant feedback to what you got Vs your intention) as a good method of learning to see.