Homepage On-Load Promo Pop-Up

I want a clean, lightweight pop-up that opens automatically the moment a visitor lands on my homepage. The overlay will show one promotional image, a short line of supporting text, and a clear call-to-action button that links to a URL I can easily change later.

Core details
• Trigger: on page load, homepage only
• Content: single image + brief text + clickable button
• Behaviour: closable by the user (X icon or outside click) and fully responsive so it looks good on mobile as well as desktop.

Ideally the code sits neatly in my existing site, uses vanilla HTML/CSS/JavaScript (or a small jQuery snippet if that speeds things up), and leaves me with one place where I can swap the image, text, and link without digging through multiple files.

Acceptance will be:
1. Pop-up appears on homepage load and nowhere else.
2. Image, text, and target link are editable in one obvious spot.
3. Overlay and close action work smoothly on all major browsers and phone sizes.

Hand over the finished files or plugin, plus a brief note showing where to edit the content, and the job is done.