Andrew, there are many ways. But if your images are on Flickr, then the easiest is probably to open your image on Flickr at the 800x534 size, as this is easy on others computers when you load it onto the forum.
Right click on it, and in the menu which pops up, select the 'Copy image URL' choice.
You then write your post in the Forum, and when you want to post that image, you click on the 'Insert Image' icon 3rd from the right of the icons at the top of the window you're writing in.
You have to select the 'From URL' option (wait for it to settle down, it won't let you select it straight away, so sometimes click it twice) and then make sure you 'untick' the box below it.
Then press 'OK'