This is more a general question about HTML/iframes than Tesseract.js, as the answer comes down to whether you can access the contents of an image (img.src) in an iframe. If you can access img.src from the context where Tesseract.js is loaded, then Tesseract.js can recognize it.

I would expect the answer is no. My understanding of iframes is that if you embed an iframe that is (1) a different domain and (2) not a website you control (so you cannot set the CORS policy), then you do not have access to the contents of the iframe. In this situation, I believe the iframe is almost like a separate browser tab.