You seem to be requesting that if someone has over 10 reputation on another site (Stack Overflow), then they should be able to post images here on Meta Stack Overflow. Assuming I am understanding you correctly, the problem with your proposal is that there is no reliable way for the system to know that someone has an account on another SE site, unless that account and their MSO account are associated.
For example, currently the network profile associated with the account you've used to post this feature request shows only your MSO account. I assume you have a Stack Overflow account. But the accounts haven't been associated, so the system doesn't know they belong to the same person.
To associate your accounts you can add the OpenID that you use for one account to be used by the other.
Once your accounts are associated, if you have at least 200 reputation on any Stack Exchange site, you get a 100 reputation bonus on every other site. So, if your Stack Overflow account has at least 200 reputation, then you'd be given 100 reputation here, which is much more than enough to post images here. (See How does "Reputation" work? for details about association bonuses.) Users who associate their accounts before any account rises to 200 reputation can unassociate and reassociate them to gain the bonus after earning 200 rep.
This does not fully address all of your concerns. A modified version of your feature request would be practical. We could confer privileges here on MSO based on rep earned on other SE sites whose accounts are associated with this one (proving that it's really you).
As others have said, a 10 reputation requirement to post images in not very demanding--two question upvotes or one answer upvote confers 10 rep. However, considering that the existence of MSO rep at all is a historical curiosity (and arguably a historically calcified mistake)--the meta 2.0 sites (meta.X where X is some SE site besides SO) have reputation and privileges based on reputation on the main site, and they work just fine.
So, is it important that a feature request like this be granted? Probably not. Would it be bad to implement such a feature? No, it would be fine. MSO is about the other sites, so it's reasonable that one way to get some privileges here would be to have them on one or more other SE sites. And if your MSO account is associated with an account that's earned 10+ rep, the chance that allowing the MSO account to post images will lead to spammers being able to post images is pretty low.
To play the devil's advocate now, though: The chance is not zero, nor even virtually zero. It's perfectly possible for a spammer to contribute slightly on one site to gain rep, and then use it to spam another site.
Finally, please note that, on any Stack Exchange site, if you post a link to an image in a question or answer (or even in a comment on that question or answer, if you clearly explain its purpose), other users can and usually will check to make sure the image is appropriate for the site and on-topic for the post, and add it for you. (After all, Stack Exchange sites are collaboratively edited.)