Two things to keep in mind:
If you link to an external image, make sure that that target image is publicly accessible. So, for example, if you link to another discussion group that requires a membership to access photos, your links will not work for most members.
Also, consider the longevity of your external links. For example, a link to a current eBay listing will stop working within a short period of time. This limits the value of your post for future readers.
For both of these reasons, you should consider uploading copies of your images whenever possible, since this provides seamless, relatively permanent access to your valuable post.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now!: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)