To put it simply Hashtags link your post to that particular word.
If someone click on #seascape for example, all posts to the Photo Wall with the Hashtag seascape will show as search results, making it much easier for visitors to find your posts again.
Hashtags can be any word you like or combination of words but no spaces or dashes are allowed for example #blackandwhite is fine but #black and white or #black-and-white would not work.
You must add the Hashtag when you first post your image, if you try and edit them in afterwards they will not work.
Hastags are always good and help people to see you posts again and again.