Use relative positioning or constraints
Use wxWindows resource files
Don't use absolute panel item positioning if you can avoid it. Different GUIs have very differently sized panel items. Consider using the constraint system, although this can be complex to program.
Alternatively, you could use alternative .wrc (wxWindows resource files) on different platforms, with slightly different dimensions in each. Or space your panel items out to avoid problems.
Use .wrc (wxWindows resource files) where possible, because they can be easily changed independently of source code. Bitmap resources can be set up to load different kinds of bitmap depending on platform (see the section on resource files).