Lizzy here, I’m a Shopify support Guru.
We do have a few different apps that could do this for you. The ones I've linked below are either have a free option or a low monthly cost. I always like to suggest checking out the reviews and descriptions to make sure you pick the app that's best for your business.
This could also be possible with a code customization. If you're using a Shopify developed theme, you have 60 minutes of free design time on your account! Feel free to let me know if this is something you're interested in and our theme support team can potentially make this change for you. You can check out our design policy here.
What sort of sales or promotions are you hoping to set up with these badges? Are you timing it with a marketing push on your social media channels?
I'd question if adding the mark/label to the image itself is the best idea.
An overlay may be better as it doesn't change the image itself. Overlays avoid those situations where the promotion is over but the old image was cached by other services (like facebook). You don't want a customer to be sad by clicking on the cached image and finding out the promotion is no longer running.
You're welcome! To get started with this option you would need to be using a Shopify developed theme and currently be on a paid plan. If you have both of those, you can reach out to our support team at any time to request small customizations to your theme. Our team will review the request and make the changes or let you know if they're unable to or need more information.
As we've spoken a bit about this customization already, I've just sent you an email that you can respond to. If you can let me know exactly what type of overlay you're looking for and where you'd like it to appear I can pass the request along for you.
Lizzy | Shopify Guru
I had a suggestion for you for adding 'On Sale' and discount labels to your product images. There are apps on the shopify app store that allow you to add product labels to your stores. You can pick the ones with discount tags. I am adding examples below that I thought would really well with your theme but there are other options which you can always check here.
In these kind of labels you can easily edit the discount value without any development effort at all. It's a simple 3 step process where you select your image, select the label and then apply the changes.
I hope this is useful for you.
If you have questions about the app, feel free to reach out to me. :)