To create an iso image using files from a particular directory use:
mkisofs -o <output_file> <source_directory>
Note that mkisofs is very powerful and can create images which support symbolic links, file permissions, etc. It can also create images which are compatible with Mac systems. Refer to 'man mkisofs' for more details. If GUI is preferred, then K3B can be used to do the same with very easy to use interface. Choose 'Burn' and select option 'Only create image'.