A ".mcpack" file is essentially a renamed ZIP archive containing the necessary files for Minecraft Bedrock Edition to recognize a resource or behavior pack. To "convert" a PNG to an .mcpack , you are usually trying to create a or a Resource Pack (like a custom texture). How to Manually Convert PNG to MCPack
: If your PNG is a player skin, tools like the cdsmythe Skinpack Creator allow you to upload your PNG and immediately download it as a ready-to-use .mcpack . png to mcpack converter
If you prefer to do it yourself to have full control over the pack's description and icons: Create a folder and place your PNG inside. Skin Pack A "