This is a public forum and our replies are read by other users who may have similar questions as the OP even if their situation and approach to starting out on Upwork is different. Hopefully, they'll read information Lanie shared and use it to be successful on the platform.
re: "To get a job on Upwork, is a cover letter is enough? Or do I also need an overview?"
By "overview," you are referring the to the editable text that forms the general description of you and your services found on your profile page.
Yes, you need to have an overview. You already have this.
You are a graphic artist. So you should keep in mind that if clients are a like me, what they are MOST interested in is your PORTFOLIO.
Personally, when I hire artists, I don't pay any attention to overview text or proposals/cover letters. I look at the freelancer's portfolio.
For me, when I see your portfolio, it looks like you just downloaded some stuff you found on the Internet. This looks nothing like the distinctive portfolios of the artists I hire on Upwork.
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.