How the hell do you guys manage to post such large images??

ThunderB
The large photos you see above on this AC forum are linked images that are hosted at other locations, while the image you have above (eagle in flight) was uploaded to Audiocircle directly. The remote locations can be any of the photo hosting sites like photobucket.com or flickr.com (both free) or they may reside on a webdomain (personal or work).
If you are on a computer (rather than phone/tablet) while browsing AC, you can mouse right-click and choose “Save image location” to see the location of that image.
Audiocircle has improved the quality and compression of the images compared to the early days, but space costs money, so the images directly uploaded to Audiocircle are limited to a certain size and the image resolution is compressed a good amount.