>>> There is also the dired-do-isearch command that searches in marked files.
>>>
>>> OTOH, when such "making text visible to search" would be implemented
>>> for isearch, then it will be used automatically by query-replace that
>>> relies on the same search functions.
>>
>> That's true...
>>
>> So we basically need some new functionality for switching to text mode
>> in image buffers when we're doing searches?
>
> Maybe isearch-open-necessary-overlays should detect if there is an image
> over point, then switch the buffer to text mode.

Here is one possible fix, but I'm still unsure if using image functions
in isearch is a good style.  Perhaps a better fix would be to add this
code to a buffer-local isearch-mode-hook in image-mode.el that runs
when isearch starts.