There are two ways of doing this.

Firstly, you can submit a photo during the uploading process, it is the optional last step.

Secondly, you can submit a previously uploaded photo by selecting it from your profile. From the grid view, you can click the more button (it looks like 3 dots) and select ‘Submit into STF Awards’, from the large popup view of the photo, you can also click the more button and select Submit into STF Awards’.