well usually tga/dss files are stored in C:\Program Files\Sony\EverQuest II\UI\[custom-ui-dir]\Images
you would access it with
SourceRect="x1,y1,x2,y2" (thats top left x1,y1 and bottom right x2,y2 of whatever you're pulling out of the file.)
SourceResource="images/[filename]"
throw that in an <image tag with size and location.
It'll help to read stuff in the
DDS/Graphics Modification Help & Info forum and look at sony's examples (the default ui)