Transparent images: choose a background before JPEG
Start with a PNG that has transparency (e.g. 800×600, alpha=true). In "compress" or "convert", set the target format to JPEG.
JPEG has no alpha channel, so the tool never silently flattens to white: enter #FFFFFF, #000 or your brand color in "Transparency background". The plan reports background.required=true plus color and rgb, documenting the exact compositing color.
With input {width:800,height:600,bytes:240000,format:"png",alpha:true,orientation:1} and background #3366CC you get output.format="jpeg", output.mime="image/jpeg", output.ext="jpg", background.rgb=[51,102,204] and output.alpha=false. Leaving the color empty raises BACKGROUND_REQUIRED.
This round emits a JSON plan, not an encoded JPEG; real encoding is untested.