Should be able to just omit the 's' on the end of that protocol (just use http: instead of https:) and include the full path like this:
You have to check the share link that Dropbox provides. If it is https (Hypertext Transfer Protocol Secure), it looks like the forum may not accept a link with that Secure protocol identifier. If you use the Dropbox option to shorten the name, it changes it to http and it does seem to work, like this example of an image in my Dropbox folder.
I hadn't noticed that problem before, but it is good to know. If you use the Preview Post option when writing a message with a link, you can check the link in that post preview.
http://dl.dropboxusercontent.com/u/3797379/DavisResearch.jpginto the URL block presented when one clicks the Image icon to insert an image inline with other reply content.