Here you can upload a new pony.
It will be reviewed and (most likely) be published on the site as soon as possible. You can help making the review easier (thus having your image published faster) if you consider these three rules of thumb:
Whenever possible, make your image square. Square images fit into the page layout perfectly, and not having to download, crop and re-upload the submitted image saves me a lot of time.
Please scale down huge images before uploading them. 500x500 pixels is about the size your image should be (in fact, square images bigger than this are scaled down during review).
Really, you shouldn't.