I wanted to find some 600×800 images for the Kindle and learned that Google Images doesn’t let you perform this kind of search now. However, there’s a small hack that lets you do exactly the same. Append the following text to the end of the Google Image search URL:
&tbs=islt:2mp,isz:ex,iszw:600,iszh:800
This will return only the images that match the exact size. Like in this example. You may replace the 600 (width) and 800 (height) with whatever dimensions you need.
Source: seroundtable.com