Even though your website is having larger images the content and the content on image is also same the users will be thinking for something new in the content about your website. Its not about duplication its about the users thinking about your website.

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Image alt-text is plain text on the Google cache (text version) so use a descriptive keyword instead of a paragraph of text.

Image-alt text can also show up inside of the SERP description If you remove the meta-description tag in your webpage HTML.

Rob,

That does not count as duplicate content.

You can even have the keyword right at the bottom of the page.

If you mean editing the images and placing the description in them, it won't be considered duplicate content since Google cannot read text in images.

You're using an alt tag for your product images? That's not uncommon. Just use common sense on what you include in the alt tag for each image, making sure that it's an accurate description of the image provided, not just a list of keywords.

image alt text will count. Google can't read your image so it will count your alt text only. Thanks