Ximage.exe is a rather simple interface. If you're interested in more fine grained control over your images, you can get that by using a configuration file. This is a .ini file that enables you to, among other things, define files and folders to exclude from the image. It took me a while to understand the value of the exclusion list. The best reason I'm aware of is to eliminate from your images very large files that aren't needed. The two best examples are hiberfil.sys and pagefile.sys. Another might be the recycle bin. The OPK documentation has a section titled "Creating Ximage Configuration Files" which provides a sample .ini file that excludes these files.
If you want to use the same .ini file for all images you can simply put it in the same dir as ximage.exe and name it wimscript.ini. Once you're done that Ximage will use it everytime. Alternatively, you can specify the .ini on the Ximage command line.