The platform you use depends on the media you're adding to your showcase.
If you're adding a video to your showcase, you can use YouTube or Vimeo, but make sure that the videos you're linking are public so camp directors can access them easily when they look at your application.
If you're exhibiting a selection of pictures or a portfolio, use Flickr. It's an easy way to get all of your pictures in the same place and once on Flickr, you can add separate categories for different projects or pieces of art you may have.
If you have an Instagram account which has your work on, you can link that too. Just make sure it is public!
Other people sometimes search: Uploading videos