You need change setting image upload in your site.

This site select menus follow 'Mysite' menu > Your site 'Folder' >> edit html/LocalSettiongs.php
## To enable image uploads, make sure the 'images' directory
## is writable, then set this to true:
$wgEnableUploads       = falsetrue;
# $wgUseImageMagick = true;
# $wgImageMagickConvertCommand = "/usr/bin/convert";

and 'html/images' folder 'Permission' set to 'Writable'.

So, you can upload on your mediawiki site via 'Special:Upload' page.
--
安冨 伸浩 (マイサイト管理人)