Eric, I realize what you issue is. You are trying to use a UNC path<iconprefix>\\xxx\xxx\xx\Photos\</iconprefix>
The client machine has no idea how to get to that. You need to use a http url insteadlike this
<layer>
<name>Weather Observations</name>
<fields>Icon_used,URL,Type,OBJECTID</fields>
<linkfield>URL</linkfield>
<linkprefix></linkprefix>
<linksuffix></linksuffix>
<iconfield>Icon_used</iconfield>
<iconprefix>http://gislap134/FlexViewer2.2/assets/images/i_</iconprefix>
<iconsuffix>.png</iconsuffix>
<zoomscale>15000</zoomscale>
<forcescale>true</forcescale>
</layer>