Uploading / changing banner images

The image used for a signature must be hosted so that you can point to it with a URL. I am using my Trainz banner hosted by Auran for my signature. I would like to continue doing this, rather than joining an image-hosting site.
So the point of my query is not how to change my signature's URL, but how to upload a new banner image that I will then use as a new signature. As I recall, Planet Auran permitted you to upload banner images, and this appears to have been discontinued in MyTrainz.