Random Quote Machine: Feedback for Portfolio-Building?

Hi all! I’ve just deployed my Random Quote Machine on github. Here is the site, and here is the source code.
I’d like feedback on what I can do to improve the app, particularly in the following areas:

1: The background image takes a while to load on some browsers. This may be because of the file size (772 KB), but I’ve put files this big on sites before, and not had issues with loading time. Are there other factors that are slowing down the operating speed?

2: As of now, the quote machine isn’t mobile-friendly. It looks very bad on screens below 600px. Should I fix it to make it more responsive, if I want it on my portfolio? I have other websites that are responsive (not to mention my portfolio itself), but if anyone has tips on how to make the #quotes-box responsive without losing the formatting, I’m all ears.

3: The twitter button connects to my twitter account, but it only gives a blank textbox, so I would have to manually copy-paste any quote I wanted to tweet. How would I adjust it so that the quote itself is automatically added into the textbox? This may be something that is too complex for the moment, but I’m curious how other sites make content that you can automatically share via other social media sites.

Beyond these issues, any general comments are also appreciated. Thanks for your time!