Show 1 image on the main index and a different image in single post

hello. i’m pretty new to wordpress themes. i did a slight modification to a theme for another blog but this time i’m doing one from the ground up. so far i’ve just been experimenting with modifying the default theme.

the goal is to make a portfolio gallery i can manage with wordpress. here’s what i have so far: https://isaidicanshout.com/

basically, i will have tiles of thumbnails and when you click on them it will go to a large version.

my question is: how can i have one image show up on the index and another image show up when you click on it to go to the single post?

alternatively, if that’s not possible, how would i go about displaying a thumbnail on the main page and the full version of the image on single post?