As Mosaic runs it needs to build temporary files. Some of these can be fairly large. By default Mosaic will build them in /tmp (a Unix directory that every user has access to). If there is not enough space in /tmp, Mosaic will not be able to display some images. One indication that you don't have enough space is that you will see the NCSA logo instead of a bitmap image in documents. The Mosaic Demo Document has several such bitmaps; if you see the NCSA logo instead of images in this document then you probably don't have enough free disk space in /tmp.
To fix this problem you can specify a different directory for Mosaic to use for its temporary files. One method is to specify the directory when you start Mosaic:
% xmosaic -tmpdir /home/users/fred/tmp