You just open the standard Photos app, press Edit in the upper right corner, select the photo or photos you want to delete (a check mark will appear on the selected images) and then select the red "Delete" button at the upper left.
When posting photos from the app, which Agarwal believes will become a major use of the app, people can quickly geotag a photo, snap a photo from the iPhone or select an existing photo from iPhone albums without even leaving the app.