Problem with finding picture url from code!!

Hello,

I am currently developing a site: http://www.alfa-kem.fi
If you take a look at this site you can see that there is a slider menu under the banner with pictures to every topic.
I would like to change those pictures and replace with my own made pictograms ( PNG files on my computer ) but I can’t locate or find the place from the code where I could do that. If I use firefox firebug I can see where they are located in HTML, but I don’t know how to find them in PHP code’s.
I am editing and uploading files via FTP.

Any ideas on this problem ?