So firstly, you should think and remind the concept of you'r site the base theme, like this site, "helping blogger's" , or some other sites, like cooking food, introducing uniqueness, some show, or do you have a car's blog, don't worry what ever base theme you have the procedure will be same.
So what exactly you have to do is, open up the most romantic search engine Google.com ;) and write you'r base theme name followed by the interesting word and click Image Searches !
For example this blog, "Blogging Guide(My base theme words) tips(my selected interesting word)" so it will be like "blogging guide tips" the selected word doesn't need to be so unique ! it can be a daily common using word but interesting now what I got from the top 10 images is as below.
What can you write on this image ? remember school day's teacher said who will attended the Image writing essay class will be beneficial in the future ! She was certainly write :D, Watch the image there are so many things you can write, you can also select other images and write on the !.
Here you got the idea before writing to the topic check out the guide's of SEO and optimization guide to enhance and make it SEO worthy.