We are often asked how to get the images to look more professional, more uniform in size site wide. Start with good images that are the same shape (all square, all rectangle)
The large image upload field is the image that displays on the "detail" page ( where the buy button is) then there are 2 options for the thumbnail field:
1) create a separate thumbnail in the size you want - this gives you more control on the size and ensures that they're all the same height/width
2) allow the system to auto-resize according to the settings you have in Catalog Setup.
When auto resizing the system uses the settings from Catalog Setup
Example: if you have the following settings
Max Image Height: 200
Max Image Width: 200
And then uploaded a large image that was 800 x 600 and opt to have the system auto create the thumbnail the following would be true
1.) The auto thumbnail height will be, at the most, 200px.
3.) The auto thumbnail width will be, at the most, 200px.
4.) With the image being 800px wide and 600px high, it's wider than it is taller, so the auto thumbnail will be 200px wide. Note that it's using the 'Max Image Width' value.
5.) The system then scales the height accordingly.
So the auto thumbnail height will be 175.8934 pixels wide, which rounds to 176px.
So the thumbnail to be 200px wide by 176px high.