Hi Leonid, here's a link to Miroslav's picture converted to 256 colours BMP...
It can actually fit into 256x192 so it can be directly opened and displayed from the Next Browser. :)
To center it properly, though, I've shortened the girl's finger by one pixel, as it was weirdly long... Though it was more obvious when zooming the picture. It feels more natural like this, but of course, the original is fine too (since it fits into 256x192 layer 2,1 it should be unconspicuous with BORDER 0 :p ).
https://www.mediafire.com/file/r2l6e1pytzlsm05/Miroslav_Bursa_1_256x192.bmp/file
Note there's no visible difference despite converting from 446 distinct colours to 255. This says lots about the ZX Next's 512 colours palette that actually contains some colours that are quite undistinguishable from one another, so I'm still trying to build a complete 256 colours palette with all the useful colours from the 9 bits full palette.
Chances are there might be less than 256 really different colours in the end (at least the human eye can distinguish). Maybe there'll only be 240-ish of them ? Which isn't necessarily a bad thing, as putting a few redundancies in the palette could help when switching from 256/320 to 512/640 pixels wide modes, as the 16 first entries are automatically used by default, and could be populated with 16 or 15 useful tints, and room for transparency.
My goal is to build a "standard" 256 colours palette using all the useful colours out of the available 512, in order to replace the unsatisfying R3-G3-B2 default palette and never worry anymore about choices. :D
In particular, the 3-3-2 palette only contains 4 greys (including full black and full white) out of the possible 8 hues, lacks lots of tans, browns, etc. and contains a useless excess of greens, limes, purples and pinks, lots of them looking completely identical.
For instance, there's absolutely no visible difference in the picture here when going from 446 to 255 separate colours. Of course the BMP is still encoded into 24 bits colours, but the Next's bmpload DOT command will automatically select the closest colours from the palette. What I don't know, though, is if it choses them from the full 9 bits palette or from the simplified default 8 bits palette.
What's obvious is some slight changes may become more apparent onscreen. That's why I'm working on my own BMP to Layer 2 loading and conversion tool, also to cope with 320x256, 512x192 and 640x256 modes, and why I need a complete 256 out of 512 colours palette, and to generate a relevant16 colours palette for the high resolution modes (though understanding Layer 3 might be a much better answer here).