I’d like to keep the main product image visible at all times, even when a variant is selected in the variant options section.
Hey Samp,
The easiest way to achieve this would be to just make the variant images the same for all variants.
https://help.shopify.com/en/manual/products/product-media/add-images-variants
… or else, remove the images from all variants, so that when a variant is selected, the image doesn’t change.
Both of the above workarounds will still allow you to have multiple images listed for the base product (ie. viewable via the product page thumbnails / image grid).
Otherwise, if you want to keep the unique images assigned to each variant, but prevent the main image from changing, then you’d be looking at custom-coding to remove the image selection functionality when variants are selected, entirely. I’d recommend www.HeyCarson.com for having that done (if no one else posts a coding-related solution for you here).
With all that said, I wouldn’t recommend making the above changes. It’s best-practice to have the images change when a variant is selected (and to have unique images assigned to each variant).
One last potential way to handle this would be via a Product Options app (https://apps.shopify.com/search?q=product%20options). You could remove the variants for the product (but keep all of the images), and then use a Product Options app to offer “options” that are essentially the same as all of the variants that you want to offer. When customers select ‘options’ generated via a Product Options app, the main product image won’t change.
Hope the above helps point you in the right direction! 
Cheers,
Stephen