In order to change the MediaWiki default logo you should do the following:
1. Create an image for your logo with dimensions of 135x135 pixels.
The recommended format for the image is .png, but other formats like .gif or .jpeg are also ok. Please note that if you upload image with different dimensions it may not be displayed.
2. Upload your logo image somewhere on your web hosting account.
For example you can upload it directly in the public_html folder.
3. Add or edit the $wgLogo line in the LocalSettings.php to contain the path to your logo
The LocalSettings.php file is located under the directory where your MediaWiki is installed. If it is installed directly in the public_html, this file will also be there. If it is installed in a subdirectory of the public_html, the file will be located in the subdirectory.
You should open the LocalSetting.php and look for a line that starts with $wgLogo. If there is no such line you should add it yourself and if there already is such a line you should edit it to contain the path to your logo. For example if we suppose that your logo is named logo.png and you have uploaded it directly in the public_html folder the line should look like:
$wgLogo = "logo.png”
If you have uploaded the logo in a subdirectory of you public_html, be sure to paste the whole path. For example if you have uploaded you logo logo.png in the skins/common/images/ directory the line should look like:
$wgLogo = "skins/common/images/logo.png"
4. If you have followed the instructions above and you still do not see your logo, please make sure that in the LocalSettings.php the image upload is allowed.
To do this open the LocalSettings.php file and change the line
$wgEnableUploads = false;
$wgEnableUploads = true;
NB: It is possible to simply overwrite the default logo installed with MediaWiki, but this is strongly advised against, as an upgrade may end up overwriting it or change the default location of this file.
If you are not a programmer and have problems changing the MediaWiki logo, ask your host for assistance. Siteground provides the best MediaWiki hosting and such requests are handled with ease.