Why do pictures in 'Text Module' expand on mobile view? How to fit them within mobile view?

By default, images using img tag in text module will expand out the pane if they're longer than pane's width. We have to set each image same width as pane or 
  • Go to 'Layout tab' in Design panel 
  • Click on 'Change default layouts' button
  • Select the default layout for standard layout or detail layout.