But unless its under size limits, it will be too large. If you host the pics on photobucket etc, then you can just use the link they have and it will show in your thread. Voila.
If you meant into a gallery in your own name on here, then cant help.
After putting (dragging and dropping or uploading - depending on wether you use their desktop client program) a picture in your "Public" folder on dropbox you can ctrl-click to get the public URL and then paste it into you post here using the