Resizing Initial Thumbnails

Howdy Themonic! I’ve used

img.alignleft.wp-post-image {
width: 260px;
}

on my Blog page to resize my post thumbnails a bit larger — but, this seems to apply after the thumbnails have loaded at 200px, rendering them more-or-less blurry when pulled on other parts of the site (like on my homepage).

I’m wondering a) what snippet and b) in what PHP file I need to edit to set this initial 200px load to something higher. Perhaps it’s a core file? I was able to find the element included in different iconic-one style sheets already, just not the root of it. (+Don’t worry, I’m already using a child theme ^^).

Thank you so much in advance for any help!