Though drive letters shouldn't be an issue with any program, signed or not, UAC may become kicky when an unsigned program tries to access a software-mapped drive (virtual drive using the subst command or a similar way, since mapped shares are a different animal).

Now, something you can try if you want, is totally disabling UAC, then you'll see icomancer works without any exceptions at all.

An application unhandled exception has raised:

System.NotSupportedException: Das angegebene Pfadformat wird nicht unterstützt.
bei System.Security.Permissions.FileIOPermission.EmulateFileIOPermissionChecks(String fullPath)
bei System.Security.Permissions.FileIOPermission.QuickDemand(FileIOPermissionAccess access, String fullPath, Boolean checkForDuplicates, Boolean needFullPath)
bei System.IO.FileInfo.Init(String fileName, Boolean checkHost)
bei System.IO.FileInfo..ctor(String fileName)
bei Icomancer.Functions.PreprocessSourceImage(String SourceImage) in H:\Icomancer\Icomancer\Functions.vb:Zeile 1985.
bei Icomancer.modEmbody.EmbodyFromOnScreenSelections(Boolean UsingLauncherControls) in H:\Icomancer\Icomancer\modEmbody.vb:Zeile 33.
bei Icomancer.My.MyApplication.MyApplication_Startup(Object sender, StartupEventArgs e) in H:\Icomancer\Icomancer\ApplicationEvents.vb:Zeile 290.
bei Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.OnStartup (StartupEventArgs eventArgs)
bei Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.DoApplicationModel()

Please take a screen shot of this and send it to icomancer_support@lavasoftworks.com so we can figure out how to fix it.