Graham UbalencyBlog This used to be a lot easier but the old method no longer works
this xtscript will do it

<!--parser:xtscript--> var $list=<xt:filelist sort_type="name" sort_dir="asc" folder="/images" template="|.file_url.#" per_page="0" filter="*.*" /> var $last=call strrpos $haystack=$list;$needle=| var $rnd=call mt_rand $min=0; $max=($last-1) var $start=call strpos $haystack=$list;$needle=|;$offset=$rnd var $end=call strpos $haystack=$list;$needle=#;$offset=$start var $image=call substr $val=$list;$start=($start+1);$length=($end-$start-1) print <img src="$image" alt="*" /> <!--/parser:xtscript-->

NOTE The way this works The first and last images (sort is by name in the script) in the folder are not very likely to be selected so add 2 specially named duplicate image files to take up those positions in the images folder for the rare occasion when the first or last get selected