The images I uploaded were well over 1600 pixels wide, at least on my computer. The smallest ones (from a Cessna 172 with a *cough* point and shoot camera) were 3648 pixels wide, and the ones from the mountain top were 4435 pixels wide…
Maybe the forum uploader or my computer is downsizing them as they are uploaded?
If so—and I know you already made your image using unsplash, so maybe this could be done next time— you could have @jamesstaddon contact the person to send them to you as an email attachment.
Hope this helps for next time!