Your Add-on Products images look blurry, how can you fix this?

We have on purpose used the shop thumbnail as it makes page load faster.

You can any way set your own image size to load in the plugin.

We have provided a setting for this image size. Where you can select an image size from all the available WordPress image sizes and set to your addon product.
You can find this setting here:
WordPress Admin Dashboard > Custom Product Boxes > General Settings > Addon image size
Select your fit size and save the setting.

Or you can also add following code to your child themes functions.php

You can use whichever image sizes you want on your site.(But large image sizes can slow down your page)

medium image size won’t slow down the page load.

add_filter( 'cpb_add_on_image_size', 'cpb_change_addon_image_size' );

function cpb_change_addon_image_size() {
    return 'medium';
}

You can refer these articles for more information on image sizes.
article 1 and article 2