10 Simple ways to improve ranking on Google Image Search

Focus on related meta data from the header into the images alt tags and title tags. For our case, each screensaver and wallpaper has it’s own landing page. Focus the image and meta data to contain very similar information to make sure the proper keywords are picked up by the Google Spider. Note: the Google Spider for Images is very different then the Google Spider for web pages.
Alt Tags – a MUST for each image and be sure to have description keywords of the image as well as include keywords that relate to the meta data in the header. These descriptions should focus on each image rather then contain a standard web phrase or tag line.
Title Tags – The title tag should be either identical to the alt tag and/or with a unique title appended to it, possible the title of the web site.
Use larger images; suggest 333px x 334px or about 10,000 pixels in size. Note that Google Images also breaks up images into 4 categories:
1. Small Images (150×150)
2. Medium Images (150×150 - 500×500)
3. Large Images (500×500 - ~1200×1200)
4. Extra Large Images (~1200×1200+)
Descriptive filenames. If the image is a picture of the empire state building name the image file empirestatebuiliding.png.
Wrap an Image with descriptive text that summarizes the image as well as uses some of the keywords and description in the alt,title, and meta tags. Some designs and layouts push the descriptive text too far down the page, focus on wrapping the image tag and text inside a div.
Create a sitemap of images with descriptive text around each image as well as a link to each image. This might be a bit over the top, but also could provide a unique viewing experience.
Focus on creating links to the image itself, such as a full size view. The link should go to the src of the image tag.
Focus on creating links to the page the image resides on
Specify the height and width of each image in the img html tag