How can I get the URL for an image resized with SetWidth().

This does NOT work:

<img src="$Image.SetWidth(150).URL" alt="$Heading.ATT" title="$Heading.ATT" />

You can't. You won't be able to use URL after SetWidth(). There is an example in the SilverStripe book on this, done by Subclassing Image.

example:

class Page_MyImage extends Image {
	
	function generateMyImageThumbnail($gd) {
		return $gd->resizeByWidth(50);
	}
	
}

Where Page_ is your class, such as Page or HomePage, etc.

Then update your has_one

	static $has_one = array(
		'Image' => 'Page_MyImage',
	);

And you should be able to use in your template like:

$Image.MyImageThumbnail.URL

You can change "MyImage" and "MyImageThumbnail" to whatever you like.

There's actually a simpler solution than subclassing. This code always worked for me:

<% control Image %>
<img src="$SetWidth(150).Link" alt="$Heading.ATT" title="$Heading.ATT" />
<% end_control %>

Don't know where that heading is coming from, eventually you'll have to use $Top.Heading, or something along these lines (ugly, I know):

<img src="<% control Image %>$SetWidth(150).Link<% end_control %>" alt="$Heading.ATT" title="$Heading.ATT" />