resizing image thumbnails

The Brink Hi, I would like the links to my posts on the main page and the category page to all be the same size. I've tried various things to achieve this end including: - cropping images to the same aspect ratio - cropping images to the same pixel size - changing the thumbnail size under "setting > media" in my dashboard Ideally I'd like to be able to control the settings at any point but for now I'm trying to get all the images to look like these. thanks TB