To like/unlike a picture on a profile, click on a picture to open the gallery. In the gallery, click on the little heart icon to 'like' the picture. - To 'unlike' a picture, click on the heart icon again.
To see who liked your picture, open 'Settings' and go to Photos/Manage pictures. Select the picture which you want to check the likes on and open it in the gallery. Next to the heart symbol, you will see the number of likes the picture has received. Click on the number of likes (Website) or the arrow symbol (App) to see who has liked your pictures.
If you want to be notified about new likes, open the settings and go to 'Notifications' (Website) or 'Push notifications' (App). Here you can activate or deactivate the notifications for picture likes.