Make sure you use descriptive and accurate use of the "alt" attribute in your code for the image. Also add your site to google image search in the google webmaster tools.
make sure your image name is optimized for the keyword you want to rank for so instead of calling it picture.jpg you would call it dog-training.jpg - then use Dog Training as the alt tag as well. done.
The text AROUND the image also plays a part, so make that relevant, e.g 'onscreen' captions should contain keywords
if you want your image to be prioritized in google image searches, based on experience, 2 factors are important. 1st is the size of the image, the bigger the image, the better. 2nd is that the keyword you are targeting should be included in the text near your image (directly below seems to have the greatest effect)