After searching during weeks for a smart variant thumbnails assign solution I found this app that works nicely with another app from StarApps, Variant Swatch King. I needed some help in customizing some details and Shubham Vats from StarApps support was really quick and helpful.
Their package of apps are very useful for customizing product variants.

Absolutely incredibly intuitive app. Works well. Amazing customer support, fixed a conflict with our theme within hours. Definitely good value for the money. Great developer.

This app is really awesome! It allows us to structure our images perfectly and show many images next to each other.

Amazing app and customer services, This app is amazing and the team supporting it is extremely helpful and makes it fit right in your website I highly recommend this app for your store.

This app has exactly the features I was googling for about a day: You can super easily sort your images for each variant and only show the relevant ones to your customers. It is very intuitive to set up (with 0 coding experience) and it also offers reliable quick support. I am happy i found this.

Great app. It works exactly as I expected it to, and really improves the user experience in our store. The support team was very responsive (keep in mind that they are on not in the US so their support hours are opposite of ours, but even so they had resolved my problem in less than 24 hours). Thank you!

Very good and fast support. Had one small customization issue that was taken care of right away. Great app.

This app works just how it is supposed to and since I have no real dev knowledge, the support team was really helpful and I was able to get it up and running in no time. I'm not sure if there are other apps that do this same thing, but I'm really glad I found and tried this one first.

App does a great job at displaying only the images relating to the variant and is exceptionally easy to set up. I have not experienced any issues with incorrect images or all images displaying against a variant. Page upload is quick particularly on mobile. Just an update - the support team is fantastic and very prompt at replying to requests. I had a very minor glitch on my site that was rectified within one hour of contacting. Can't recommend highly enough.

Excellent app that works perfectly for our products that have 50+ images. Always super responsive & helpful too. 100% Recommend!