I am working on my store and I found out that there are certain images being called but not available in files section thus arising 404. You can check this in waterfall tab of this report https://gtmetrix.com/reports/koolishirt.com/54jlaKcn
I want to find the source from where they are being called so that I might delete them and they are no longer called
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This is an accepted solution.
The theme you are using has a colour swatch for product options. It tries to load background images (the ones you are seeing 404's for) but because it also has a regular background colour set in the style, you don't see the error on the page.
In order to fix that, you would either need to remove the inline styles on those colour swatches (can't say where that is in your code because I am pretty sure this is a commercial theme) or you will have to upload the missing PNG images to your Assets folder.