The source attribute for the tag is misspelled in the code you pasted above. It needs to be src.
<!DOCTYPE html>
<html>
<head>
<title>Changeup test</title>
</head>
<body>
<img src="tick.gif" alt="tick" onmouseover="this.src='tick2.gif'; "onmouseout="this.src='tick.gif';" />
</body>
</html>