Here’s the clause you need to be aware of, and what you’re effectively giving to National Geographic if you upload ‘any’ content to their site - important bits in red-italic-bold.
5. For clarity, you retain all of your ownership rights in material you upload, comments you post, or other content you provide to the Site (“User Content”). By uploading User Content, you grant National Geographic (which includes its subsidiaries, affiliates, joint venturers, and licensees) the following rights: a royalty-free, worldwide, perpetual license to display, distribute, reproduce, and create derivatives of the User Content, in whole or in part, without further review or participation from you, in any medium now existing or subsequently developed, in editorial, commercial, promotional, and trade uses in connection with NG Products. National Geographic may license or sublicense, in whole or in part, to third parties rights in User Content as appropriate to distribute, market, or promote such NG Products. An NG Product is defined as "a product of National Geographic, a subsidiary, affiliate, joint venturer, or licensee of National Geographic, in any language, over which National Geographic has "Editorial Control." For the purposes of this Agreement, "Editorial Control" means the right to review, consult regarding, formulate standards for, or to exercise a veto over the appearance, text, use, or promotion of the NG Product. You also agree that National Geographic may make User Content available to users of the Site who may display and redistribute it in the same way that National Geographic makes all other Content available.
Incredible. What this means is that you're giving your work to National Geographic, for free, for ever. Please don't do this.