Permanent Image resize

Hi ,
I’m working on my site with the theme grid-a-licious from suprb.I wanted to resize the image smaller to fit the 1st page of my site.

When i make my image smaller inside wordpress itself it only appears to be downsize when inside the actual post.But at the main page its still its original size .You can view this on my site https://www.silentt.net.I cant manually edit it outside wordpress everytime coz i will have other contributors to the site.

Where and how do i edit this in the coding?will it be in the php file or the css file?best if i can keep a standard size for evry post at the main page.

please enlighten me.

Many many thanks