There are two sets of images to consider: the default WP Media images and the theme-generated images.
WP Images: The Settings > Media screen allows you to change the maximum dimensions of images that created. When setting a thumbnail size, the image will be either cropped or proportional, depending on the setting. Medium and large images will keep the dimension proportions, taking the maximum width and height set here into account. Note that these dimensions are the maximum values, not the actual size of the image. This means (1) if your original image is smaller than the medium or large width and height maximums, that size will not be created, and (2) if your original image is larger than either the width or height, the image will be created using whichever dimension is maximized first and then scale the other dimension to fit.
Theme Images: The theme also creates thumbnails when images are uploaded. These default sizes are:
small 160×160
standard 320×320
medium 520×245
large 720×340
However, when these images are created they are scaled to fit those actual dimensions. If the original image size is less than these dimensions, the image is scaled up; it it’s larger it is scaled down. These dimension are controlled in the alx_setup function in functions.php. Read more here.
So, if you leave the WP media sizes at their defaults, here are a couple of examples of what will be created when you upload an image:
1. Original image 3000×2000 will create the following:
original: 3000×2000
WP thumbnaill: 150×150
Theme small: 160×160
WP medium: 300×200
Theme standard: 320×320
Theme medium: 520×245
Theme large: 720×340
WP large: 1024×682
2. Original image 180×200 will create the following:
original: 180×200
WP thumbnaill: 150×150
Theme small: 160×160
WP medium: not created because original is smaller than 300×300
Theme standard: 320×320
Theme medium: 520×245
Theme large: 720×340
WP large: not created because original is smaller than 1024×1024
Hope that helps.