Image sites are all about the authority of the site i.e
- Back link quality and relevancy
- Age of the site
- Social interaction
- Regular freshen content
- Regular added content
You only need to search "Image of Flowers" and "Flowers Backgrounds" to see that Google does in fact rank 'thin content' on certain topics and searches, of course adding text content would help but of course if people want just images then simply adding content to satisfy the search engines would affect user experience because all people want is the images.
Google Image Search (Unique Images Preferred)
Unique images work best as Google has the ability to see if images are already stored somewhere else online, you can check this by dragging a image from Desktop into Google search and it'll scan for duplicates and well if they can tell that they can easily detect duplicates which ultimately may lead to less Google image search clicks).
With LESS you need MORE
Generally with LESS you need MORE, but with this said if everyone in the same category has LESS then you don't need as much of MORE. Social media is most likely your best bet as pictures can go viral which will help your SEO from all the social mentions and because more people see it they could even link using their blog or something.
Adding additional markup may help slightly but generally it's not treated as much as everyone would hope, off the page signals are still the most contributing ranking factor.
Suggestions that won't gimp the user experience and promote better search rankings
- If possible and economy viable use a CDN to host the static files such as 'JS, CSS, JPG, PNG and so on' (Google does reward fast sites).
- Adding a comment system might improve user experience while making the content that little bit less THIN and can be treated as a page update which is a big plus to 'Freshen Content'.
- Ensure that the ALT describes the image well, a short sentence is ideal i.e `Pink Sunflower in a farmers field' rather than 'Sunflower'.
Many other ways on what are the best ways to increase a site's position in Google?