Google doesn't want to let you download images directly. But you can just take a screenshot, so... why not just maintain the image quality?
this is a greasemonkey script.
- run a google search for images.
- Click one to be brought to its details
- refresh the page (just noticed this, going to fix)
- you know below the image, on the left, it shows a globe icon and normally the name of the domain where the image is? It now says "direct". Click that.
- You'll be brought to the image, directly. Rather than the page that hosts the image. Download as usual.