Minecraft paintings have very specific dimensions: 1x1 paintings are 64x64 pixels; 2x2 paintings are 128x128. Crop your PNG to these dimensions (multiples of 64). Save it as my_painting.png .
If you try to open your newly created MCPACK and Minecraft says "Import Failed": png to mcpack converter
Converting a PNG image into an .mcpack file is a common task for Minecraft players looking to create custom or texture packs . An .mcpack file is essentially a compressed folder that Minecraft recognizes as a resource or skin package, allowing for one-click installation. How the Conversion Process Works png to mcpack converter