What CFL said, but in a more concise format...
skin1/customer/head.tpl
Open that, near the top you should fine something along the lines of this:
<div class="logo"><a href="{$catalogs.customer}/home.php"><img src="{$ImagesDir}/logo.gif" alt="" /></a></div>
Make sure the img tag points to the correct one. If you dont see that, check the div as it may be in there.
Also, you may need to adjust the width, size, padding and if part of the logo is cut-off make sure that the div or any ones above it in the tree dont have
overflow: hidden