Still pondering whether to do a post on resizing and cropping images (that one's for you,
@Tabascocat lol) but there is one Google search tip I forgot that might be helpful. In the
www.google.com/images search bar, type a search term, then a space, then 'imagesize:<width>x<height>. This method will generate images with the width and height you are looking for--if there are any out there matching them. Here are some examples:
Halle Berry imagesize:200x200
Halle Berry imagesize:300x300
Halle Berry imagesize:400x400
Halle Berry imagesize:400x400
My personal size preference has been 400x400 in this thread. The dimensions deliver good size that fills the screen with only one post when scrolling or when the page jumps to it.