Changing Invoice image and the mouse over text

[admin]

The Store default logo is oscommerce.gif or oscommerce.png found in the admin/images directory - replace either one or both if you have them with your logo just name oscommerce.gif or oscommerce.png

Also change the store/images/store_logo.png - replace it with the same named file but with your logo image.

I have just changed the oscommerce showing when mouse is run over it!
includes/header.php
look around line 19 for

  <td colspan="2"><?php echo '<a href="' . tep_href_link(FILENAME_DEFAULT, '', 'NONSSL') . '">' . tep_image(DIR_WS_IMAGES . 'oscommerce.png', PROJECT_VERSION) . '[/url]'; ?></td>

Change to   <td colspan="2"><?php echo '<a href="' . tep_href_link(FILENAME_DEFAULT, '', 'NONSSL') . '">' . tep_image(DIR_WS_IMAGES . 'oscommerce.png', 'Web Global Net') . '[/url]'; ?></td>

Should do the trick