I have an image from my article on 2nd place in Google images for one common 2-word health term, and on 1st place for related 3-word term. The site with the image has PR = 0. I used alt tag and used the term below the image also. The term is mentioned only 4 times in the article, but one word from the phrase is used about 50 x. There are 21,400 images for that term. I have 2 images from another (PR4) site on 1st G.images page (1st and 11th place) for 3-word term. 5,050 images for that term. Both sites are only some months old and articles with images are online for few weeks.