Images: auto resize to page width

How to Make Images Auto Resize to Fit Blog Post Area - Free Online Web Tutorials

Theme > Customize > Advanced > Add CSS

.post-body img {
  max-width: 100%;
  height: auto;
}

.post-body img {max-width: 100%;  height: auto;}

If it doesn't work that means the same styles already exist in the template.
You have to override the existing styles

.post-body img {
  max-width: 100% !important;
  height: auto !important;
}

Πώς θα προσαρμόσετε το μέγεθος των εικόνων στα posts σας
CSS: .post-body img(max-width:100%)
How to Resize Blogger Photos Automatically

http://www.designerblogs.com/how-to-resize-blogger-photos-automatically/