Should I make it centered or add some space to left. I think centering it will look good.

The space for the icon is reserved with padding, so it doesn't have its own enclosing element. Plainly centering the icon won't work in this case.

Where can I find the image posted above ?

ContentTranslation/modules/tools/images/ contains the icons.

Where can i find this in mediawiki ?

@Abhinand,
You can find it in the mediawiki extension ContentTranslation. If you do not have it, go to gerrit.wikimedia.org go to the project section, go to list, search for the extension, and clone it. Then, use the path given by Nikerabbit to find the location of the image. :)
MtDu

@Nikerabbit,
What do you think is the best positioning? I can override it in the specific x-icon class. Does background-position: left 5px center sound good? (I haven't gotten around to updating my vagrant instance, so I can't test a this moment. :p)
Thanks,
MtDu

Perhaps @Pginer-WMF knows? My simplistic guess would be to have equal amount of whitespace on all slides.

Can I make it as background-position: left 5px center ?