If you use social networks to promote your images, I suggest you an interesting site.
This site is a list of hashtags usable in various areas.
I found this site searching for something similar in the web, and I find it very interesting, so I want to suggest it to you.
The operation is simple: go to the site, where NOT to pay anything, choose the list of tags best suited to your image, selecting and copy them in the comments of your image and the game is done.
The site is this one!