Blog grid thumbnails different sizes

Hello,

Could you, please, help me figure out why in grid mode (shortcode [blog]) the thumbnails for the different publications are not the same size? Check the screenshot.

Thank you!

P.S. Is it possible that the reason is I don’t upload pictures per post (i.e. using the media dialogue box on each post), but rather I do this via FTP and in bulk, then use a plugin called “Media from FTP”? Perhaps that’s the reason WP doesn’t generate specific thumbnails sizes.

Also, why does the theme pull the whole picture, instead of just the thumbnail? This is a waste of resources, as my pictures are often over 2500px on the long side. How can this be changed?