AppleScript: Get list of 15 most recent files in directory
Change the line,

    set imageName to the file name of imageItem


to 

    set imageName to the name of imageItem


and it should work.

EDIT:

Couldn't help but notice that you are not quoting your file names. If any of them contain spaces, then you will have problems. Best to also change

    set concatFiles to concatFiles &amp; " " &amp;  imageName


to

    set concatFiles to concatFiles &amp; " " &amp; quoted form of imageName