Just make sure you set the paths to the support files and the image folder correctly:
For example, in the page_prerender event, you could put code like this:
gallery1.FilesPath = "~/DotNetGallery_files"
gallery1.FolderPath = "~/CustomerImages/samplefolder"
The ~ operator is a little know asp.net shorthand for the root of the web application, so relative to the top of your site structure. So for example if the site is
www.mysite.com, "~/DotNetGallery_files" refers to
www.mysite.com/DotNetGallery_files.
Hope that helps.