To navigate in file tree, click on a folder. Second click closes it. To download, click on a file you llike. Download speed: Max. In near future there will be a backups for skins (ID1, ID2, ID3 and so on)
Here is all files from gta3.img archive. If you have messed up your game and can not find backup from above, then you can search and download them here. Enter a search term, like turismo.dff and hit Enter. Right below the search box you’ll see search results. You can search files by typing in partial name of the file, like turismo. A click to file name downloads it, click to file path right under the name opens up containing folder and highlights it.