The background shapes in the Mint theme are actually gradients and not images, but regardless they can be disabled with a little CSS. You can create a new theme component, add it to the Mint theme, and include:
#main-outlet::before,
#main-outlet::after {
display: none;
}
You can then add your own background image with a little more CSS. First you’d want to add the image to the uploads section of you theme component:
Then in your CSS you can reference the image’s variable name like this:
body {
background: url($blobs);
}