When merging together css file from multiple folders the image paths should be rewritten to local the files.
For instance consider the following mash config:
<File Path="jqtouch/jqtouch.css" />
<File Path="themes/jqt/theme.css" />
<File Path="cutom.chirp.less" />
themes/jqt/theme.css has references to images within its local img folder:
background:url(img/toolbar.png) - relative to themes/jqt/
When merged and minified the paths remain the same, so images will not be found.