An image I uploaded to flickr isn't making it's way to the forum by way of URL. Does the insert image feature not work with HD photos or is this flickr's fault?
Flickr can be tricky. This is particularly true on my smartphone. You need to find the URL of one of the actual image files, not the URL of the Flickr page containing the image.
Okay, so I just now visited your Flickr site. I picked one of your pictures, and then I found in the pop-up menu that is activated by the "..." button, "view all sizes". On that page, I chose the "Medium 800" size, and clicked on that. That showed me a version of the picture that's about the right size for this forum. Then I right-clicked on the picture, and chose "Copy Image Location". That last bit is browser-specific; I use Firefox.
Now I'll paste that copied URL into this message, inside an [img] tag:
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Ffarm4.staticflickr.com%2F3784%2F9194685051_6a8f79f1a0_c.jpg&hash=719b62b56785141a5f1045b3f8051275197c80fe)
[img width=800 height=450]http://farm4.staticflickr.com/3784/9194685051_6a8f79f1a0_c.jpg[/img]
Alternatively, as the owner of the pictures, Flickr may provide you with some sharing code in a few different formats. I think the one labelled BBCode is compatible with this forum. You can just copy and paste that, and you'll get your picture complete with attribution and a link back to your Flickr site.
Another way is to do a Google Images search for the caption you labelled it with in flickr, then choose "View Original Image"; that should take you to just the picture. The tricky part is that, half the time, your search query will return pictures that are only near the one you want on the flickr page, but not the one you actually want. It's almost like flickr associates the captions with the wrong pictures.