Very good job! Below are some comments where there is room for improvement:
In the "Register", "Login" and "Upload Artwork" pages, it takes some time for the form to appear. I like the animation, but I think it should start instantly, and not after a couple of seconds. Also, maybe it doesn't have to come all the way from the bottom, but instead just move up a bit and fade in? Anyway, I digress.
Uploading an artwork feels like it should be a modal, and not a completely new page. The user should be able to quickly upload the art and select it from the list without having to change pages. It's not a big deal and you didn't lose any marks for this, but it would be an improvement.
User Benefit and Complexity: it is a tough decision, because we must choose between "Complex enough", or "Not complex enough". And while it breaks my heart to choose the latter despite your website looking really solid, I feel like there is still room for more functionalities. Maybe you could focus on the portfolio creation aspect of the site. In your proposal, it felt like you were going to be more creative with this, but currently there are only limited customization options available. Maybe turn your site into a full-fledged Linkedin-like art portfolio, where the user can maybe add different sections (e.g., Skills, allowing users to input several of their skills and let other users on the website upvote them for that skill; among other sections), and just add more customization in general. I feel like this is the heart of the webapp, and more attention should be given to this feature.