Hi
I don't typically change the size of my images on different breakpoints, and you only need 1 or 2 breakpoints. Most of the time I only use 480 as a break point.
For anything above 480 I scale my images to 786 X 260. 800 x 300 also works and gives you a nice number to remember. For 480 and below I scale my images to 480 X 160. The minor stretches are barely noticeable.
Go through the document here
http://wysiwygwebbuilder.com/forum/view ... 10&t=63817. Helped me a lot. You can take a look at
www.xocomputers.com. It is using Layout Grids and has some pretty large images, you can view it on both desktop and mobile, it only has 480 as break point, but it scales fairly well.
Wysiwyg is really incredible once you learn all the tools.