yes.. you should always use alt tags and if possible you can use title as well but i think only alt tags is sufficient ...
I think Google is our best friend who's able to give us every single knowledge on SEO. I get the following link from Google which will be helpful to you. http://websitetips.com/optimization/images/
You can use alt tags in the html codes. Use other image submission websites like design firms and given above.
Google shows images in SERP as well. So unique name given to images using alt tag will help you a lot...
rename your image to somewhat that contain your targeted keywords, convert it to png (I think Googlebot loving the png image :s, cmiiw), and use alt tag for each image