Image size for full width row

Are you using the image as a background? Background images on the demo site are quite large. This ensures that they will display well on all displays.

See here:
http://themes.kadencethemes.com/pinnacle/wp-content/uploads/2014/08/demo_img_05.jpg

If you’re using it in an image widget or something similar, then usually something that’s 1200×1200 will be large enough. Generally speaking, the larger the images, the better.

You can use a tool like this one to compress your images so that they don’t impede on your site’s speed:
compressjpeg.com

Hopefully that is helpful