Prompt for password

I have an asp.net 2.0 internal web application that uses authentication
mode="Windows" and identity impersonate="true" with a userName and password.
Our photos (mostly jpg files) are stored on a 2nd server and we have a web
page that displays them in a DataList. If I go in as administrator there is
no prompt for credentials. If any user goes in they are prompted for
username and password and I cannot figure out why.

I looked at the access rights to the folder where the images are stored and
they have rights for EVERYONE and also NETWORK SERVICE. What am I missing
for security that is causing a prompt for credentials (p.s. other non-image
documents work fine). Thank you.

David