What App does it offer to open? An Email app maybe, by attaching the photos to a new email to share them? A social media app (Facebook, Twitter, Instagram..) and creates a new post with the pictures?
Are you trying to share the photos by text? you Gallery app doesn't have a built-in capacity to share photos to someones else's Gallery app. Sharing a photo has to be done either by email, text, bluetooth.. or any other app.