I tried using GalleryFiles as the image directory but still got the same error.

 

However, I did found the problem.  The page had LivePerson chat code on it which was writing it's own <form> tag to the page (outside of the <form runat="server"> tag... there were no nested forms) and once I eliminated that then the upload worked fine. 

 

I tested further by adding an empty <form></form> tag to the page higher in the sorce than the <form runat="server"> with the DotNetGallery control and the error reappeared.  Then I moved the empty form to just before the </body> tag in the source (under the <form runat="server"> </form>) and the upload worked again.  I'm guessing that somewhere in the DotNetGallery code it assumes that the control is in the first or only form on the page and references form(0)