Open your album and then click on the photo you wish to share. The photo will enlarge and three dots appear top right. Click on the three dots, select the middle 'share' icon and choose how you wish to share your photo: email, facebook, twitter, pinterest, whatsapp (only on mobile) or different types of links.
Some forums or sites require a smaller size photo, so you can choose from: 'L' Large 1080 px wide, 'M' Medium 720 px wide, or 'S' Small 360 px wide photos and BBcode.
Embedding a single photo
Usually you will be able to do so using BBCode on a forum. If not you can use the 'html' tag.