Hello,
Please avoid using spaces and special characters in the image file names. For example, instead of using the name "BLOG MAIN IMAGE 750 X 397.jpg" use it with the dashes "BLOG-MAIN-IMAGE-750-X-397.jpg".
After renaming the images (you can do this in FTP or in Joomla Media Manager), you should re-select the blog images from the Joomla article.
Regards,
Milos