C# - Associate Image To Webcontrol Without URL?

1st) Server retrieve an image from Database, in the Database i've only the path, then i encapsulate the image in a XML and i send this XML to the client.

2nd) The client decapsulation the img, so the client has an array of bits with the image. it's not allow send the path for security.

Now i need associate this image with my webcontrol img.

i do something like

ImgUrl = "Handler.ashx?num_doc=13"

and it works, but really i need do something like:

ImgUrl = "Handler.ashx?num_doc=" + num_doc;

where num_doc it's a http parameter recieved from a page. I'll try to put the ImgUrl from code but it doesnt work, just show a path.