Ah, the file structure came out a bit confusing this time... You probably extracted the skin in both resolutions into your skins folder. You only need to extract the folder inside either the 4096 or 2048 sub-folders, or the skin won't load correctly. Sorry about that.
Thank you! Kunos provides a preview builder that you can use. Inside your ...steam/steamapps/common/assettocorsa/sdk/dev folder, you'll find the ksPreviewBuilder. Run it, and exactly follow the instructions (including copying some files) as stated underneath the drop down list, otherwise it won't work. Once you have it set up, it should work without issues. Good luck :)