You could use ibrickr to browse and add/delete files but i think it uses jailbreak. I am liking winscp which is sort of like an ftp program and i cant remember if it requires jailbreak. I think i got the tutorial from hacktheiphone.com
anyway, if you figure that part out... you might be interested to know...
I tried deleting the file called SBDockBG2.png which is located in /System/Library/CoreServices/SpringBoard.app/
(see
http://hacktheiphone.com/iphone_multimedia.html)
Result was (had to reboot iPhone to see results) that the dock turned solid black with the icons showing on top of course, which was not what I had hoped for. I was hoping to be able to see more of the wallpaper. I would like to see the dock area transparent except for the icons. I think the dock bar just gets in the way of the wallpaper myself. I didn't like the black so i just put the said file back and rebooted for now.