While searching in Google image search and if you like to search particular size and aspect ratios of photos there is simple trick which is given by Google OS blog.

Searching according to MP ( Mega Pixel)

Normal Google Image search doesn’t allows to search according to MP (Mega Pixel) size. Now you can search and restrict to 2 MP, 4 MP, 6 MP, 8 MP, 10 MP, 12 MP photos or larger photos.

http://images.google.com/images?q=National+Geographic&imgsz=2mp

http://images.google.com/images?q=National+Geographic&imgsz=4mp

http://images.google.com/images?q=National+Geographic&imgsz=6mp

http://images.google.com/images?q=National+Geographic&imgsz=8mp

http://images.google.com/images?q=National+Geographic&imgsz=10mp

http://images.google.com/images?q=National+Geographic&imgsz=12mp

Aspect Ratio Filter

Now you can search Google image search with aspect ratio filter by adding imgar= query string in the end of Google image search URL.

Squared photos
http://images.google.com/images?q=panda&imgar=s

Nearly squared photos
http://images.google.com/images?q=panda&imgar=ns

Tall photos
http://images.google.com/images?q=tour+eiffel&imgar=t

Wide photos
http://images.google.com/images?q=niagara&imgar=w

Panoramic photos
http://images.google.com/images?q=niagara&imgar=xw