You need to go to the Flickr page of the photo in question, click on the share icon, then in the box that comes up go to the BBCode tab, select the size you want from the lower drop down box then copy the highlighted link in the box above it (Ctrl-C on a PC, something else on an Apple) then just paste it straight into the TM box where you are typing the message, don't use the insert image button.
2017 Drumtochty Games-2203 by
Calum Fraser, on Flickr"
The bit that you paste in from Flickr will look like the text in the code box below;
{[url=https://flic.kr/p/UUzQMD][img]https://farm5.staticflickr.com/4253/34725612543_aba89f5b5e_c.jpg[/img][/url][url=https://flic.kr/p/UUzQMD]2017 Drumtochty Games-2203[/url] by [url=https://www.flickr.com/photos/39263697@N08/]Calum Fraser[/url], on Flickr}