Also these days I guess even 5MB images won’t hurt anyone.
I don't really see the point. The largest image I have posted, is still under 2 MB. There's no point embedding larger images. If you really need a higher resolution, add a link.
I can try asking @theymos to adjust the image proxy to 3MB, which would even give more freedom. 
That doesn't help. I forgot why you increased the limit from 2.5 MB to 3 MB, but this image should have been 1.4 MB so none of this is a solution.