Vlog theme will automatically generate all image sizes to fit each layout you choose to display. Our maximum content width is 1100px so with images that are 1100px wide you can make sure you covered all layouts. Also, if you are not using layouts without sidebars, images with 800px width will be enough. Don’t worry about larger images as theme will resize them properly, just avoid to upload small ones.
The only exception is cover area which obviously requires large images to display properly. It is up to your taste, but our suggestion is to upload images that are at least 1500px wide which will work with default theme settings. You can also check the documentation topic regarding settings for cover area to learn more about setting its image size.