What needs to be done for a model to be posted?

I recently tried adding in Stardust Speedway from Sonic Generations, but due to the following problems that I don't fully understand, it was rejected. 


Your model was not accepted for the following reasons:

• The textures are an unacceptable format or there are duplicate/junk files. Generally, all textures should be in .PNG format. (There are some exceptions to this rule, but this is not one of them.)
  
  Other image formats either take up a lot of space due to lack of compression (.BMP and .TGA), have limited support (.AIFF, .DDS), or lose quality in compression (.JPEG).
  
  Proprietary image formats such as .PSD are also not allowed.
  
  
• JPG format permanently damages the quality of images. You will have to re-convert the original textures. A game screenshot is not good for a big icon. It should be a custom render of the models you are submitting. The model itself is fine.
  

If anyone would mind telling me the problem's meanings and how I could've done better, please do so. If I just can't, then I will just keep them for my own usage, I guess

Due to the ZIP/RAR being too big, can't post it for anyone to really look over.