Changing regenerated photo

Hi there,

I regenerated all of the images on my blog when I changed themes. It grabbed the featured image for the popular posts which are on the front page of my blog. I would like to change the image used. This probably sounds like a dumb question but I’m not very tech and can’t seem to figure out how to change this.

Please help.

I don’t want to include my blog because I get too many spammers messaging me through my contact form. Thanks

https://wordpress.org/plugins/regenerate-thumbnails/