Method 1: If you use Dropbox but prefer to post your images inline, so that people aren't shuffled off to the Dropbox website to view them, Dropbox don't make it easy for you but there is a cunning trick you can employ to make it happen.
1. Get your Dropbox link however you normally would, for example using "Copy Dropbox link"
It should look something like this:
2. In the post you are about to make, position the cursor where you want the image to be, then click the "Image" button
3. Paste that image path into the dialogue box that appears, and (this bit is important)
4. Click "Insert", finish editing and save your post, and your image should be displayed inline.
For the technically-minded, the "BBCode" to insert the image looks like this*:
And here it is:
* The sharp-eyed or terminally nerdy may notice an umlaut over the i in the closing "img" tag; this is to fool the EM image proxy, which unhelpfully changes my example URL into the proxied version rendering the example completely useless. Please do not attempt to use an umlaut in your "img" tags