Hi pensive612,
This is a post to clarify things for others that hit this problem -

I think seethrou has hit the nail on the head.
Basically to make SlideshowPro work correctly:
Put your xml file in the root of zen (where your index.php file is)
Your xml should be like so:

<gallery>
<album title="My Title" description="My Project"
lgPath="images/YOURFOLDERNAME/gallery/album1/large/">
tnPath="images/YOURFOLDERNAME/gallery/album1/thumbs/"
<img src="yourPictureName.jpg" caption="Your Caption" />
</album>
</gallery>

Hence the lgPath and tnPath need relative links
BUT the img src just needs the picture name.

The html code for the flash is as follows (note relative links in bold)


<object classid="clsid:d27cdb6e-ae6d-11cf-96b8-444553540000" codebase="http://fpdownload.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=7,0,0,0" width="360" height="400" id="slideshow" align="middle">
<param name="allowScriptAccess" value="sameDomain" />
<param name="movie" value="images/YOURFOLDERNAME/slideshow.swf" />
<param name="quality" value="high" />
<param name="bgcolor" value="#ffffff" />
<embed src="images/YOURFOLDERNAME/slideshow.swf" quality="high" bgcolor="#ffffff" width="360" height="400" name="slideshow" align="middle" allowScriptAccess="sameDomain" type="application/x-shockwave-flash" pluginspage="http://www.macromedia.com/go/getflashplayer" />
</object>

As fas as I can see - you don't need to alter the HTML to add
<param name="base" value="." />
<embed base="." </embed>

The reason I can say this is because I have just created all of the above in a SlideshowPro project and it is all functioning correctly. I hope this helps someone in the future.