for newer OS2 themes you can try to add the code discussed here into a "custom liquid" section or block depending on theme support.
Some themes also have a "custom html" section.
Hey this post is very old but I need help adding text overlay to some of my images
That is a separate topic which varies wildly in solution depending on theme though there a many posts that deal with these types of customizations found through searching the forums.
If you need to just buy that overlay customization or support for this threads customization contact me at email@example.com with store url&storefront password, theme name, and usecase details.
thanks for the response - appreciated. @TheGiftingLady in the end found an app (free!) that did what i want by accident - was looking at email newsletter options and found this https://apps.shopify.com/automizely-messages within the app there is an option called conversion tools, here you can set custom banners on the product images - New, Back In Stock etc and choose which products to add theme to - works really well.