Well, wxWidgets use different graphical libraries on each platform.
For Mac, there's wxMac using
Carbon and in development wxCocoa using
Cocoa. According to the wx documentation, neither of these libraries support adding bitmaps to menus.
On Linux, wxWidgets uses GTK+ to display all elements like buttons, text fields, and so on. This means the programmer writes the code for wxWidgets and wxWidgets uses GTK+ to display everything.
On Windows, wxWidgets uses the Windows API. Applications written with wxWidgets look exactly like other Windows Application.
wxGTK (Linux and GTK+)and wxMSW (Windows) support images in a menu.