The issue you are running into comes down to how Pinterest handles image downloads. When you download an image directly from the main Pinterest feed or preview thumbnail, Pinterest compresses the file into a low-resolution format. Even if you crop it or edit it in an app like Photos, you are only enlarging a low-quality file, which makes it look blurry on the MacBook Neo's high-density Liquid Retina display. Upscaling can only go so far, and if you start with a low resolution image for your wallpaper you will be disappointed.
To fix this and get crisp, high-quality wallpapers, follow these steps:
Grab the True Full-Resolution Image
Never download right from the Pinterest grid or main pin page. Instead:
- Tap on the Pin to open it, then look for the external link button (the original website it came from).
- Click through to the original source website.
- If you must download from Pinterest directly, right-click the image, choose Open Image in New Tab, and look at the URL. If the URL contains something like /originals/, it is the highest quality version available. Right-click and save that version.
Also, the current version of macOS Tahoe is 26.7.1. Based on your post you are running an older version, so the first step is to update your Mac to the current version.

See: Update macOS on Mac - Apple Support