Easiest way to insert photo from Flickr in interesting size is copying the link prepared by Flickr in BBCode and deleting from this all besides link in second part e.g.
[url=https://flic.kr/p/2qJob9C][img]https://live.staticflickr.com/65535/54300874412_4c8e7d05f0.jpg[/img][/url][url=https://flic.kr/p/2qJob9C]PUSER Truck Max[/url] by [url=https://www.flickr.com/photos/183062961@N07/]K P[/url], on Flickr