I uploaded a few images, and the thumbnails on the website aren't consistent.
This is happening because the sizes that you are uploading are not the same ratio. The system will take the image you upload (let's say 1600 X 1200), and then resize them to the size needed. The system only uses the width to determine what will fit in the space.
So, let's say your thumbnail is 400 pixels wide, then the resulting image will be 400 X 300. If you upload an image 1600 X 1000, the system will resize to 400 X 250. If you want your images to all be the same on the site, then you have to upload them all at the same size (or more correctly, the same ratio because 1600 X 1200 will be the same as 2000 X 1500 - a 4:3 ratio).