yeah, I think that way (if you WANT a preview) you have to upload a preview image too, and not just let DA resize it for you.
I don't let DA resize my full-view pictures XD I usually just resize things myself on photoshop or whatever, but it may be to your liking, so I don't know...? when I submit to DA, I just choose the first option in the 'full view size' box, called 'Original size (__ x __ pixels)' (the spaces are whatever your pixel size is for your picture, before you uploaded it to DA).
but if you WANT a preview image, I think you can only do that when submitting certain file types. (it used to not matter, but I guess they changed it.) look here:
FAQ #79: How do I submit Artwork?