If you are embedding an image CTA on your non-HubSpot site, you may need to apply additional CSS declarations (such as width: 100%;) within your stylesheet to make the image CTA responsive.
What is retina optimization?
Retina optimization makes your CTA's image more crisp when viewed on your site pages and in emails if your recipients view the CTA in modern versions of Apple Mail and iOS mail; this is not supported by Outlook or webmail services like Gmail.
Retina optimization is supported when you upload an image that is at least two times the size necessary for the CTA, similarly to how retina support works for all images across the internet. If you're creating a CTA that will be 400px by 100px, the uploaded image would need to be 800px by 200px to support an @2x pixel density display. CTAs support adding up to a 4x version, in this case uploading a 1600px by 400px image. The CTA will automatically be rendered on your website with a srcset attribute serving the 1x, 2x, 3x and 4x version where available.
If the image is uploaded at the size the CTA will be rendered at (e.g., a 400px by 100px image is uploaded for a 400px by 100px CTA), this will not have retina support.