To answer your questions:
1) All thumbnails are auto-generated and get adjusted to a 3:2 ratio upon uploading.
2) As long as the photo looks good to you upon uploading, you may upload pictures of any size (up to 4 MB). Should the editors need a high-res version of your photo, they will contact you via e-mail.
If anything else comes up, please note that there is a special e-mail address set up for these types of questions: YFPcontest@wernerpublishing.com
Thanks and good luck!