To replace the Joomla logo at the top of the page, carry out the following:
1 Assuming that you wish to replace the Joomla logo with an alternative graphic, upload the new graphic file to templates/[template name}/images, ensuring that the file has a jpg or png extension.
2 Also ensure the graphics file is no larger than 298 x 75px (px=pixels) However you can change this size in the template.css file (see point 4 for access details)
3 Before uploading the graphics ensure the templates/[template name}/params.ini file permissions are changed to 777
4 To complete the change, go to Template Manager>[select Template Name]>Edit CSS>Select temaplte.css, and replace the joomla graphics entry at approx line 70, witht the name of the new image, and SAVE.
Have a look at your website and the change should now be seen.