I normally apply two rules to adding images to a page: size and size.
The first size is the "physical" size in kilobytes. The majority of the pictures on my sites
are less than 15 kilobytes (with most of the sales pictures settling in
around 8 kilobytes).
I like getting bigger pictures from my clients so that I can do more with the editing. If I get a picture
that has already been compressed, it is much harder to do color and lighting corrections.
A typical example was when a client sent me a picture of some Anatolian Shepherd puppies for sale, and
the original picture was 835 kilobytes and was 1620 by 1200 pixels wide and tall; After doing my work on
the picture, it appears on the net as a 8.5 kilobyte picture that is sized to 250 x 188 pixels.
This brings me to my second "size:, which is display size (in pixels). I ask
that my clients not crop their pictures either (because that really limits what I can do with the
pictures). If a picture is cleanly cropped, it is nearly impossible to change the aspect ratio and get
it to the same size as the other pictures.
If you observantly scan my sites, you might notice that I try to make all of the pictures
the exact same size. I do that for a variety of reasons.
The main one is to control the flow of the page.
HTML web pages are not designed like powerpoint slides or word documents; On those, the images and
text are all statically placed and do not move. The nature of HTML is that you use tables to get the images
to line up, but you have to account for different screen resolutions by your customers (internet explorer
does not have a zoom feature like you will see in the aforementioned applications.
If you go to a site that doesn't care about that, you will see short pictures in the same row as tall
pictures and wide pictures in the same column as narrow pictures. This results in a page flow that is uneven
to the eye and often displays poorly on different screen resolutions.
So, I try to make
the images the exact same height and width and I have pretty much settled on pictures that are either
250 x 188 (which is a factor of the standard dimensions like 1024 x 768 or 1600 x 1200) or I make landscape
pictures that are 500 pixels wide (and however tall I crop them).
Having guidelines for picture specifications has streamlined the development of my sites, giving them
a somewhat standardized feel -- in the same mold as a catalog or a brochure (which is an apt comparison
for most of my web sites).
See some examples below of how different sizes of a similar picture set can not look as good when they
are sized the same.