I don’t think it’s that. Like this image from Reuters served by Cloudflare works:

On the other hand if you try to embed an image served on Reuter’s /resizer/ endpoint it won’t work embedded since they have some authentication process for it.

I don’t think embedded images are being proxied through Lemmy. It’s likely the browser trying to fetch the images directly which fails in some cases.

Edit:

Does the image load in my previous comment?

Yeah it does.