So what’s happening is the images are only displaying because they’re within your content, and the full content is showing for those posts.
The posts without an image are showing the excerpt (which strips HTML, like images).
So the solution:
1. Set your images as “Featured Images”.
2a. Remove the more tag from your posts so the excerpts show.
2b. Use the custom excerpt metabox provided by WordPress to add a custom excerpt.
I think those are the only two options to fix #2 (and #1).
Let me know if this makes sense or not 🙂