I recommend using the curl library when communicating with a web service. I don't know if .NET has native functions for this.
http://curl.haxx.se/libcurl/dotnet/
Their PHP example: http://reg.imageshack.us/xmlapi.zip
function uploadToImageshack($filename) {...
ImageShack has an API:
http://reg.imageshack.us/content.php?page=developer
Using the XML API provided should be easy to integrate.
Refer to php code: http://elliottback.com/wp/archives/2008/06/01/using-the-imageshack-xml-api/
The code provided, should be fairly easy to modify and convert to...