diff --git a/include/wx/image.h b/include/wx/image.h
index 44a63769c2..ca9cc6a3c1 100644
--- a/include/wx/image.h
+++ b/include/wx/image.h
@@ -104,7 +104,7 @@ public:
     operator wxBitmap() const { return ConvertToBitmap(); }
     wxBitmap ConvertToBitmap() const;
 #ifdef __WXGTK__
-    wxBitmap ConvertToMonoBitmap( unsigned char red, unsigned char green, unsigned char blue );
+    wxBitmap ConvertToMonoBitmap( unsigned char red, unsigned char green, unsigned char blue ) const;
 #endif
 #endif
 
@@ -142,7 +142,7 @@ public:
                   unsigned char r2, unsigned char g2, unsigned char b2 );
 		  
     // convert to monochrome image (<r,g,b> will be replaced by white, everything else by black)
-    wxImage ConvertToMono( unsigned char r, unsigned char g, unsigned char b );
+    wxImage ConvertToMono( unsigned char r, unsigned char g, unsigned char b ) const;
 
     // these routines are slow but safe
     void SetRGB( int x, int y, unsigned char r, unsigned char g, unsigned char b );
diff --git a/src/common/image.cpp b/src/common/image.cpp
index ef7c6c123e..8778bb4289 100644
--- a/src/common/image.cpp
+++ b/src/common/image.cpp
@@ -464,7 +464,7 @@ void wxImage::Replace( unsigned char r1, unsigned char g1, unsigned char b1,
         }
 }
 
-wxImage wxImage::ConvertToMono( unsigned char r, unsigned char g, unsigned char b )
+wxImage wxImage::ConvertToMono( unsigned char r, unsigned char g, unsigned char b ) const
 {
     wxImage image;
 
@@ -1084,7 +1084,7 @@ bool wxImageHandler::CanRead( const wxString& name )
 #if wxUSE_GUI
 
 #ifdef __WXGTK__
-wxBitmap wxImage::ConvertToMonoBitmap( unsigned char red, unsigned char green, unsigned char blue )
+wxBitmap wxImage::ConvertToMonoBitmap( unsigned char red, unsigned char green, unsigned char blue ) const
 {
     wxImage mono = this->ConvertToMono( red, green, blue );
     wxBitmap bitmap( mono, 1 );