when you put an image on your blog, you need to add alt=" keywords " tag after your <img src="source". This is like keywords but for your images. That will work.
You may very well want to check the following article : Optimizing Images For Search Engines. Seo for images is different from the usual textual SEO, there are certain points worth considering to make it easier.
Add ALT and TITLE tags to images descrbing the content of the images. This will optimize your images to appear in google image search. Also name the image file names with keywords so they can be found easily in the search engines.