Concatinating a string with XML-tag

|
+-+Pictures (folder)
| +-1.jpg
| +-2.jpg
|
+-+XML      (folder)
 +-XML-file.xml
 +-XSL-file.xsl

The XSL file will read <id>-tag in the XML-file and create a document that
shows the images in the pictures folder.
For instanse, if the tag is this <id>2</id>, the picture 2.jpg will show.

NOW TO MY QUESTION:
"./../Pictures/<xsl:value-of select="number(id)"/>.jpg" gets me the
string, but how do I insert it as the src?
And also, is it possible to check if the picture exist?

Thnx in advance!
Jimmy