The ISO9660 standard does not allow names with other than A-Z, numbers and underscores. No matter which tool you use, if it just creates a plain ISO9660 filesystem, it will likely convert all filenames to respect these restrictions, and possibly even to fit into 8.3 filenames.
Support for other characters in filenames, as well as longer filenames and other filesystem features not found in ISO9660 is usually achieved by using extensions to the standard: in the UNIX world, there is Rock Ridge. Microsoft has Joliet. Enabling one of those will likely give you what you want (I'm not sure about Joliet, but Rock Ridge has always worked with unicode filenames, even if I don't know for sure how are the names encoded).
mkisofs, for example, you use
-R to enable the Rock Ridge extensions (
-r for slightly different defaults) and
-J to enable Joliet. Look for similar options in
isomaster. You can do well with Rock Ridge. Whether you want to enable Joliet or not depends on whether you plan to read the image under Microsoft OSes. (You can have both at the same time.)