Using dropbox or something similar you can copy the link from there and add it to your post. If you add an image to your dropbox you can right click it to get the URL for it, then in the C2 forum you choose the IMG in the editor and paste the url between the tags. Like so:
[img] <ADD URL HERE> [/img]
[/code:171ylhkb]
For the CAPX you need a certain amount of reputation before you can add a link, as I don't think you are allowed to. (Not sure if you can add images either but if you cant you can do it the same way as you would the capx. shown below)
But you need to add the url so it doesn't look like one. An example could be like this:
[https://www.google.com/]