@stefan
there is a new plugin by Zarbuz for Magicavoxel! https://github.com/Zarbuz/SchematicToVox/releases

You can enter any image under 2048 * 2048 (4096 * 4096 will works but will be truncated)

Each matrix will be 126 * 126 *126 and image automatically trimed !

Little bad side it's line commands! So for Windows 10 : Logo windows + R and enter the following line
if your image PNG is on the same folder (and with 255 colors max - black colors up palett are error by my fault! ;)
C:\Users\Pilou\Downloads\SchematicToVox-v1.0\SchematicToVox.exe --i toto.png --o totovox

will be the same for an object format schematic (Minecraft) not yet tested but i believe there will be only one color!

my test with a little image of Verve! ;)