changing thumbnail resolution for bloglovin

Disclaimer: I’m a total newbie and have fumbled my way through to get my blog to look somewhat the way I want, but I’m sure the CSS is pretty ugly.

My problem: I’ve got it listed on bloglovin (through feedburner) but the image associated with new posts comes in at a very low resolution. (Not sure if this link shows what I’m talking about as I’m on the mobile site right now, but it’s been wrong in the subscriber’s feed when the thumbnail is larger.) I am assuming this is because the thumbnail for my theme is 150×150, even though the images I upload are much larger and appear just fine larger on my blog. Can someone tell me if there’s a way to make sure my thumbnail image quality stays high when it’s burned on my feed and thus appears on bloglovin? Thank you!