Variant swatches - Variant images help?

Shopify Partner
74 4 9
Help? Not sure if I'm missing something obvious here but I want my color swatch feature to change the variant images instead, I changed the background-image part of the code to this but it keeps coming up with the no image sign, when there's defiantly images as variants set in the product back end:
background-image: url({{ variant.image | img_url: 'small' }});
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