Blog post images can only be displayed at a maximum of 500px wide. When an image is uploaded to the Media Library it is automatically ‘crunched’ meaning that it is duplicated into different sizes: Thumb, Medium, Large, Full etc. Please use the smallest width available above 500px (unless you do not require full width) and then beneath the ‘Link URL’ box, click on the ‘File URL’ button. When you click on the image within the post, it will now automatically open the full size version in a separate window.

N.B. If your image then appears squeezed, go back to ‘Edit Post’ click on the image preview then on the ‘Edit Image’ button. Under ‘Advanced Settings’ manually change the width to 500px, leaving the height blank. Click on ‘Update’, then ‘Update Post’ and voila.