How to auto resize image

Option: Change CSS Code (this trick work for me)

If you’d like every post image to appear smaller, you can change the default image width from 750px to your preferred size. Do not make the image size larger than 750px, though.

1. From your Blogger Dashboard, go to Theme.

2. Click the orange Customize button to view the Blogger Theme Designer.

3. Click Advanced and then select Add CSS. (You’ll need to scroll down to see it on the list.)

CSS Code Snippet

Copy the code below into Blogger’s Add CSS box.

[css].post-body img, .post-body .tr-caption-container {
padding: 5px;
max-width: 750px;
}
[/css]

Change 750px to the preferred image width. Do not choose a number larger than 750px because that would make your images wider than your post width.

3. Click Apply to Blog to save your changes. You might need to clear your browser history to see your changes.