onmouseover not working with React.js
You need to capitalize some of the letters.
<img src="/images/profile-pic.png" height="100" onClick={this.onClick} onMouseOver={this.onHover} />
Both the answers above are correct, but you need to bind these method to the class context too!
<img src="/images/profile-pic.png" height="100" onClick={this.onClick.bind(this)} onMouseOver={this.onHover.bind(this)} />