From those points you mentioned, though, those images are all part of a master page. Do you think that could be causing the issue?
HA! I got a gut feeling we are getting 1 step closer.
For the record, I'm not familiar with ASP.NET2.0, therefore my solution could be wrong (again), but this is how I would tackle this problem:
Review where is the master page is locate, and where are the aspxs are located.
I believe you might have 1 folder keeps the master page, and other folders keeps the aspx (or ascx) pages.
for example, lets say you got 2 folders in your asp.net web application: /Main, and /Src. Inside your /Main folder, you got another folder call /Pic where stores all the pictures.
Suppose your master page is locate under /Main, and your aspx pages are locate under /Src. it make sense for Master Page to call its picture by doing: string url = "Pic/blahblah.png" right?
But if it is your aspx call the pictures through master page, will "Pic/blahblah.png" still gonna work? You don't have a /Pic folder under /Src though.
Again, I could be wrong, but this is the approach I would follow:
Move the image to the same folder as the aspx page (or master page) see if it loads, if it does...slowly move the picture 1 folder level at a time back to the folder where it suppose belongs to. And to adjust your path accordingly.
See if that helps...