For future reference, use the Code button and put code between those BB tags when posting code in the forums.
First of all, you're missing an equal sign in the second line:
<img class="header"
Second, web pages work from top to bottom. If you want something in columns or to go left to right then you'll need to say so, otherwise the browser assumes to put things underneath. If you want text beside an image, you'll have to use a table and put the things in separate cells.
<table border="0" cellpadding="0" cellspacing="0">
<tr><td>
<img src="YOURPICTUREHERE.jpg">
</td><td valign="top">
Your text here.
</td></tr>
</table>
I made two td cells, which is next to each other. I put valign="top" in the second one so that the text is at the top of the cell, otherwise it'd be vertically centered. Valign means "vertically align", so you know.
By the way, you misspelled arial in your header_text class. And if you're on a PC you can always code stuff in Notepad, you don't have to use Dreamweaver.