Triton lite- Change Homepage Pictures?

Hello! I’m using the theme Triton Lite, which displays many posts on my blog’s homepage, represented by an image form the post. I’m a bit confused as to how the theme chooses which image to represent the post with though. I thought it used the first image I uploaded to the post, as that seems to be the case most of the time, but every now and then it chooses a different image. How does Triton Lite choose which picture from your post to display on the homepage? Is there any way to change this picture?

Thank you,
Windie