I was messing about with WebImage and found that either webImage or the html <input type="file" ...> is fussy about the case of file extensions.
Specifically, I wrote a little code to let the user browse the PC's files and select an image file to upload then have WebImage save it off to the images directory of my website.
Turns out that when the filename is "myImage.GIF" the result is a null value from WebImage.GetImageFromRequest()
If the filename is changed to "myImage.gif" everything works fine and the file is, indeed, saved. I haven't tried it with the other extensions, but this could be a possible explanation if others find inconsistent results from WebImage.GetImageFromRequest()