There are two ways to submit photos.
1. If you have not submitted photos yet, you'll need to set up a photographer account first by clicking on the "Sell your Photos" button on the top of the site. This will guide you through setting up your photographer account so you can get paid and at the end, ask you to link your Instagram account to start uploading photos.
2. If you have already gone through the photographer setup, then click on the top right dropdown menu and click on "Submit Photos", this will take you to the upload page.
3. You can also access your photos by clicking on Manage Photos in that same dropdown and clicking on the red "Submit Photos" button in the center of that page.