How can i change the text into a image (tumblr)?

Come up with a image that is 600px wide. Get the image hosted somewhere.

Then you go where you customize the site and find this line. It should kinda look like that.
<h1 id=“blog_title”><a href=”/”>{Title}</a></h1>

Now we add the image. For example:
<h1 id=“blog_title”><a href=”/”><img src=“http://imgur.com/I7Rvr.jpg”></a></h1>

That should all be on the same line. Obviously you would need to use the URL for the image that you want.