[Zope] getting a value in a string

[dirk@xs4all.nl]

Hi all, today I had my first zope day. I read some
documentation, did the (elvis) tutorial, then tried
to make something simple. (I thought) 

I want to make some simple picture album. For now it seemed
best to create a folder for the images inside the folder
which holds the DTML code. Displaying all the available
pictures works. But I actually want to display a thumbnail
which is a link to the bigger picture and I want a link to
the real full size picture so anyone who wants to can
download it. Alle sizes have some common part in their name,
let's say img_100.jpg, img_100_640x480.jpg and
img_100_thumb.jpg. Okay, now the problem. 

I can get the file name of the image that I display, but I
can't get in a variable. I can display it but I can't assign
it to some string. And I need it to be a string so I can
manipulate it. 

How can I get a value in a string? 

Dirk