Letting JNEXT fetch one

You do not have to find an image yourself. On the first run, with no SD card configured, JNEXT offers to download the official ZX Spectrum Next distribution image:

No SD-card image was found at ~/.jnext/.

Download the NextZXOS official distribution image and install it there? [y/N]

In the graphical version the same question appears as a dialog titled jnext — SD card image. Answer yes and JNEXT will:

1. Download the official distribution archive and unpack the SD-card image from it, showing progress as it goes.
2. Prepare a working copy of that image, keeping the downloaded original untouched.
3. Boot from the working copy.

Both files are kept in ~/.jnext/sdcard/, and together they take about 2 GB of disk space. The download happens once; every later run reuses it and starts immediately.

Answering no leaves JNEXT with nothing to boot from, so it explains the problem and exits.