Dupe headings - usually in the theme. Common issue is that for some reason Shopify themes like to wrap the header logo in an H1 tag, which results in a dupe. Probably not a major issue, but I wouldn't say makes sense from a semantic HTML pov.
To change you'll likely need to get hands on and edit the Liquid/HTML in the theme. It can be risky and a bit of a learning curve if you don't know what you're changing, or pay a dev to fix it for you.
For the images, I wouldn't bother with ALTs if they are just icon/components or backgrounds of the content.