Image Compression

I’m using Freelo theme and I want to prevent the theme from compressing my images when creating thumbnails etc, so I want the quality to be full (as I’ve already minimised the file size with tinyjpg). I’ve seen a similar problem in another discussion here but was unable to follow the instructions, maybe some instructions need to be theme specific?

I’m a novice at tweaking backend but am hopeful I can give this a go.