This is what I have done in the past...

The HEADER area also allows for a BACKGROUND IMAGE, which can be defined in the stylesheet:

#logoWrapper

So you could create your "logos" as a background images and add:

--------------------------------------------------------------------

#logoWrapper {
background: #fff url(../images/logo-english.jpg) no-repeat left top;
}
... to a language stylesheet for ENGLISH (english_stylesheet.css)


--------------------------------------------------------------------

#logoWrapper {
background: #fff url(../images/logo-french.jpg) no-repeat left top;
}
... to a language stylesheet for FRENCH (french_stylesheet.css)


--------------------------------------------------------------------

#logoWrapper {
background: #fff url(../images/logo-spanish.jpg) no-repeat left top;
}
... to a language stylesheet for SPANISH (spanish_stylesheet.css)


--------------------------------------------------------------------

THEN...

Just create a 1 pixel X 1 pixel transparent GIF for your logo.gif image (as it will no longer be needed as the actual logo, but is still needed as an image is "expected")

If you use JPG for your logo image, then choose a "neutral colour" for that single pixel, and save it as logo.jpg

If you use PNG, then you can do the same as for GIF, bu making the PNG transparent.