It all comes down to personal taste but for me there are some details that should be common sense - skinning wise.
1. Aligned graphics make a much more professional impression. The design from the front could move on to the hood and fenders.
2. I am no fan of any kind of coloured exhaust pipes. They should look like what they are - metal.
3. You should try the GIMP. It is a free graphic program, in many ways similar to Adobe Photoshop I think. It will take a while to learn what you can do with all the different tools. Most things are done easier than in MS Paint and the quality of your skins will most likely improve drastically.
Just use your favorite search engine and look up "GIMP".
comments xDD 