Each and every cover letter should be creafted to the project, not a cut/paste cover letter. I write to the poster, to reach them and let them know I am offering my services and how the process works. I recently won a project because I told the reader in the letter that I wasn't going to waste their time with a letter saying how great of a designer I am. I got the project 5 minutes after sending that letter.
"I have downloaded your required files and viewed your cityscape image and am here to offer my services to you and a quick turnaround. I'm not going to waste your time with a proposal that tells you how awesome of an illustrator I am, I'm here to offer you my services as your request. Let me know and I'll start. I'm ready to start immediately, if your ready to hire me."
So write to the viewer and be short, concise and add samples. Make them WANT to hire you.