OK, I'm afraid I can't help with "that old error message" from Windows Photo Viewer as I've never used that app in my life. But this sort of illustrates my point that you need to explain, step-by-step, what you are doing and what doesn't work. Because what you seem to say here suggests that images you downloaded to a PC yesterday can no longer be opened by the PC, and if that is what you mean then that also means that the problem is on the PC rather than the phone. And if that isn't what you mean, then you need to say clearly what is happening.
It's a common problem with these forums: you know what you mean, and you know the context to everything you say, but we only know the words you type. So a lot of stuff that you assume or take for granted we have no way of knowing. Like in your first post you didn't mention Windows at all, so I assumed you were wanting to attach photos to emails on your phone. The first mention of Windows was in the second post, and even there it wasn't obvious that you weren't copying to the computer just as a way of compressing. But after your third post I'm wondering whether you were always wanting to send the emails from a PC and were copying the photos to there in order to attach them (in which case it might be relevant to know which email software or service you might be using). And whichever (if any) of those is correct, unless you say we can't know, and hence may be looking in the wrong place for the answer. And that's just one example, there may be many other things that are relevant but which we don't know.
One question I should have asked at the start: how large are your images? Because I'd expect an image from a 13MP phone camera (Moto E 2020) to be a few MB in size, which should not cause problems for any email service these days (e.g. my Pixel 2, 12MP camera which I keep on its maximum resolution settings, photos are typically 3-4.5 MB, with the largest being about 6.5 MB). So if you add a lot of those to an email I can see some servers rejecting it, but one or two shouldn't be an issue.