Just do
<img src="{@fullLength}">
It's called an attribute value template.
Don't do this:
disable-output-escaping="yes"
It can get you into all sorts of difficulties. Fortunately in your case it doesn't cause any problems because there are no special characters in your text and therefore no escaping would happen anyway.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference