Hi, the path you quoted is actually pointing to the delivered skin repository directory. These skins aren't "live"--they are the source from which you can copy skins for each site. mojoPortal delivers update skins to this directory only, so that any customized skins you have won't be overwritten during upgrades.
The skins that are live for each site are located under Data\sites\[Site#]\skins\[SkinName], and that's where you would make CSS and image changes to update them. So to make a change to your skin, you should place your new images in (for example):
Hope that helps,