OK. Thought of another idea because it looks like you have 1 image per post.
But it's not automatic and will require some fussing with html code:
copy / paste, not (necessarily) writing it yourself. But also some editing of the html…
Temporarily switch your blog to private; change theme to "Duotone"
View blog, then choose "archive" on the navigation menu.
You should see a page with thumbnails of the first image on each of your posts for the year. (there is an option to see thumbs of first images per month as well)
Open a window to "view source code" and use your browser's "find" feature to locate the list of images (find the name of your first picture).
The source code will be an unordered list.
Copy the code from
Change your theme back to the one you are using, then make a new gallery page, and paste that html into the html editor.
It will be an index list of the first images of all of your posts.
You can then delete all the html code for the unordered list.
Your images will all be side by side, so you can edit them in the visual editor (or if you know html, use that) to add some space between them.
Lots of fussing, but then the bonus is that your Gallery page will also have links to your original posts.
And of course, each time you update your blog, you will want to add the new picture, by copying the link from the source code on duotone.
Not sure this is what you were hoping to hear…
Maybe someone else has another idea.