Possible hack? Images replaced on proxy server

[David Hodges <dhodges () outermost com>]

I am responsible for several ASP and ASP.Net web sites that are hosted at 
an independent ISP. These sites were developed for a corporate client which 
has its own corporate network and firewall, completely separate from the 
ISP where these sites are hosted.

The other day, an employee of this corporation was surfing our site from 
within the corporate firewall and found one of our images was coming up as 
a porn image! Another employee was able to verify this.

Then we found that other images were coming up with no content, or as 
horizontal bars of color.

These problems are not occurring outside the corporate firewall;  and the 
source images, on the server at the ISP, are fine. Only people behind this 
firewall see these bad images.

I suspect someone has hacked the corporate proxy server but I have no way 
to know for sure. I am in somewhat of a panic because naturally it does not 
reflect well on my little company to have porn images coming up on sites we 
develop.

I renamed the image in question and changed the IMG tag in the html, which 
fixed the problem for the time being. But I am worried about future.

Would a META HTTP-EQUIV="Pragma" CONTENT="no-cache" tag help?

What else can I do to prevent this, and, what can be causing this?

Thanks,
David