WordPress has three default sizes for images you place on a page or post:
You also have the option of setting the image to it’s full original size. You can, however, change those default sizes. This is helpful because you can set the large size to the width of your site’s center column, medium size to an image you might wrap with text in a post or page and thumbnails for preview or head shot images.
Of course you can always modify the size once you’ve uploaded it, but it makes things simpler when the defaults match the parameters of your site. Here’s how to change the defaults:
1. Login to your site and go to the Dashboard.
2. Click on Settings at the far left then Media under that.
3. You’ll see this screen where you can set your new sizes. You can make the dimensions a square (500 x 500) even if the image you upload is a rectangle. WordPress is smart enough to know the largest dimension of the new image and adjust the other dimension accordingly to maintain the proportions:
I’ve set the large image for this site to 750 x 750 which is about the width of my content area. Remember that the images you already have uploaded will retain the old settings. Only the new uploads will reflect the new sizes.