First of all: someone have made something to the macrolibrary. May be corrected by webmasters, I think. http://kmeleon.sourceforge.net/wiki/index.php?id=MacroLibrary

That's is why I'm going to paste this macros here and because last time I make a mistake to the wiki too ... The macros are in spanish, my language, but I think isn't a mistake to translate for every language ;-) I added a "&sourceid" statement too. I mean that when you use this way to search at google you are telling them that you are using K-meleon (the same as when you use Opera). I think that is the best ;-)

Well here they are:

# Búsqueda con F4 de imágenes en google

PromptSearchImg{
opennew("javascript:imgGoo=prompt('Buscar imágenes para:',''); while (imgGoo==""){imgGoo=prompt('Buscar imágenes para:','');} if (imgGoo==null){location.href='about:blank'} else {location.href='http://images.google.es/images?q=' + (imgGoo) + '&sourceid=K-meleon'} ");
}

# Búsqueda con F3 de texto en google

PromptSearchTxt{
opennew("javascript:txtGoo=prompt('Buscar en Google:',''); while (txtGoo==""){txtGoo=prompt('Buscar en Google:','');} if (txtGoo==null){location.href='about:blank'} else {location.href='http://www.google.es/search?q=' + (txtGoo) + '&sourceid=K-meleon'} ");
}

Ups, I forgot

Add the above to the macros.cfg

And this to the accel.cfg:
VK_F4 = macros(PromptSearchImg)
VK_F3 = macros(PromptSearchTxt)

Note that you may have already configured F3 to "ID_NAV_SEARCH" so you only may need to replace "ID_NAV_SEARCH" by "macros(PromptSearchTxt)" if you want to use the macro above instead of the default way to search at google.