All of your posts must have a featured images, these images will appears in the slider, homepage news boxes, sidebar, etc…
Theme cropped all uploaded images to fit its places in the theme. There are two ways of cropping – with standard functions of WordPress or by using built-in script bfi thumbs. To enable script cropping enable it in Theme Options Global Enable\Disable Enable resizer script (enabled by default). Then script create folder
/wp-content/uploads/thumbs_dir/ for thumbnails. On some servers script can not do this, so create it manual and set chmod 777 permissions to folder.
Sometimes, cropped images are not very good for you. So, you can try to disable cropping. Check Disable crop in resizer script
If you want to use standard WordPress thumbnail cropping and you have any problems with the size of old images which uploaded before install the theme you need to use Regenerate Thumbnails Plugin or AJAX Thumbnail Rebuild Plugin to regenerate thumbnails with suitable sizes.
Also theme has built-in lazy load script for thumbnails. Enable it in Theme Options Global Enable\Disable to increase speed of site.