>> +(defun image-mode-isearch-filter (_beg _end)
>> +  "Show image as text when trying to search in the image buffer."
>> +  (when (and (derived-mode-p 'image-mode)
>> +             (image-get-display-property))
>> +    (image-mode-as-text))
>> +  t)
>
> This would switch to text representation for all images, though.  Is
> that what we want?  Hm...  perhaps?

It makes sense to do this even in binary formats when
the user wants to search the text of e.g. EXIF tags, etc.

> So that's a good solution for isearch, but I don't think we'd want that
> for search/replace.  Possibly.

This is what OP wanted (unfortunately, mails to his mailbox bounce back).
But we could install the patch anyway.  The only problem I noticed
that the previous patch shows isearch highlighting at wrong places
because there is a need to wrap the call to image-mode-as-text
in save-match-data (alas, global match data bites again).