Reverse list (by post format instead of by theme) here:
Upload all the images from your computer and click Insert Gallery. Then go to Media > Library, hover over the image you want to display, note its ID number in the status/progress bar of your browser. Back to the post editor, switch to HTML, change the gallery shortcode to this, if you want a thumbnail:
[gallery include="ID NUMBER HERE"]
Or to this, if you want a full-width image:
[gallery columns="1" size="full" include="ID NUMBER HERE"]
The gallery post format is supported in some themes only, and the result will be one image on the main posts page but all the images when you view the single complete post. The alternative I suggest works in all themes, and it will give you one image in the post on both the main posts page and the single post view.