Icons
Help:Contents provides details about using graphics.
I formerly used the Icon Extension before this functionality was included in newer versions of MediaWiki.
Installing Icons
In order to get the Oxygen icons into this mediawiki system, I installed the icon set (well actually I think it came as a dependency with some kde4 application) which provides the compressed svg files. Then I copied the compressed files to a temporary location where I could decompress them and import them. Using the import tool provided by mediawiki, I batch imported the ~800 images. I didn't categorize them, which had I known about categorizing images I would have done first, rather than later. If you want to categorize your images on import, just add the category to the comment.
find /usr/lib/kde4/share/icons/oxygen/scalable/ -name '*svgz' -exec cp {} /tmp/ \;
gunzip --suffix=svgz /tmp/*svgz
for i in /tmp/*\.; do mv $i ${i}svg; done;
cd phase3/maintenance
php importImages.php --extensions=svg --user=Freephile --comment='Get the whole set at http://www.oxygen-icons.org' /tmp/
Creating Favicons and Application Icons
You can use Inkscape as a tool to create your web and application icons.
Resources: