Using Firefox browser, you can right click on the image and select (1) "View image" or (2) "Copy image location" (or something similar, My Firefox is not in English).
If you do (1), then look at the url in your browser to find out where it is.
If you do (2), paste the url in your notepad or wherever you need it...
They are defined in css. so that you can find them out more easily if you use something like the web developer toolbar of firefox, which is great to get css's separated from sites you are
viewing, and edit them right there seing the changes on the fly.
It would allow you to search for text next to the image, and find out that there is a section called: tikipedia-logo, etc. The image/s are like: tikipedia/logo.png, which would mean ./yourtikiroot/styles/tikipedia/logo.png
Ask again if still in doubt...