I hope it's OK to tack on to this old thread ...
I too am having a problem with the tracking image and the opt-out link as well. They used to work but I just noticed they are not working any more. I'm not sure when/why they stopped working.
to answer your questions:
1. Other images do not appear
2. The Opt Out link does not work
3. The tracking image is a GIF not a JPG ... but like I said, it used to work
4. I am not using a 3rd party URL module
When I looked at the source of the image, I noticed the portal alias was part of the URL. Copy I copy paste the URL in a browser it doesn't work. If I remove the portal alias then it does work.
Doesn't work (goes to the home page):
http://mytrooponline.org/gssrq/Default.aspx?TabID=97&ShowImage=True&OptInEmailID=56c9be44-0bf4-4393-8f4c-45bc073676c2&NewsletterID=beecbb5a-a76f-498e-8891-fffa20554305
Works (shows the image):
http://mytrooponline.org/Default.aspx?TabID=97&ShowImage=True&OptInEmailID=56c9be44-0bf4-4393-8f4c-45bc073676c2&NewsletterID=beecbb5a-a76f-498e-8891-fffa20554305
The difference is the one that works does not have the portal alias in the URL (gssrq). I did recently add a new portal alias (gssrq).
I would appreciate any ideas or suggestions. Thanks!