“Parameter is not valid” exception from System.Drawing.Image.FromStream() method

I got a hard time with the Image.FromStream method in my website. The code below works perfect on my computer. But when I uploaded it to the test server, it always gives me "Parameter not valid" exception.

if (!afuImageFile.IsUploading && afuImageFile.HasFile)
{
    System.Drawing.Image imgFile = System.Drawing.Image.FromStream(afuImageFile.FileContent);
}

the afuImageFile is an AsynFileUploader control in Ajax Tool Kits. afuImageFile.FileContent is a HttpInputStream. I guess I need to add some permission to some folder. Can anyone help me?