…that blog posts can be any size?
Well, I guess they can be ANY size you want, but if you make them too short, WordPress will take the caption text from your second image and turn it into additional textual content in the preview. So don’t make a super short blog post.
But maybe you could make a blog post that’s just long enough to make the Content Management System (CMS – that’s what WordPress is!) happy, and not rip off your photo’s caption for content?