You may submit content for any section on FlashStuf. There are many ways that you may submit content to be reviewed for addition to the site.
Submit your content through the forums. Register there and post a link to your file in the "Submit Content" section of the forums. If you don't have your file on the internet already, you may use the FlashStuf uploader to temporarily store your file.
Or sign up for a FlashStuf account and then follow this link to request to be an artist. Once approved, you'll be notified and be able to start submitting content.
Special content, such as Adoptable Pets and Styles may not be possible entirely on your own means. For example, FlashStuf has it's own means for tracking pet stats and a stylesheet that you'll want to download to have something to base yours off of. For these special files, simply contact me directly.
If your content is accepted, you'll be notified through PM on the forums or email, depending on how you submitted your content.