Hopefully this works..
<center><="http://ff14sp.com/ffxiv-signature-generator/"><img src=http://ff14sp.com/tools/lodestone_data/imgs/Alexandrious Dragorath Selbina 6 .png></center>
Delete the bold section and see what happens. If nothing, then just use
<img>http://ff14sp.com/tools/lodestone_da...s/Alexandrious Dragorath Selbina 6 .png</img>
and replace the <, > marks with [, ] marks (brackets if it doesn't show up properly).
Hang on, gonna try some tests.
EDIT:
It worked for my signature when I used
http://ff14sp.com/tools/lodestone_da...ina%206%20.png
in between the [ img ] [ /img ] tags (without spaces though).
Hmm it won't post the entire URL on the post, but if you click the link it will bring it up and you can copy the URL from the address bar.
