XnConvert, an image convertor with GUI (some time ago i told you about how badly DOpus converts images with default chroma subsampling, so i finally found a work around with GUI hehe).
If i use direct open then it will spawn one window per opened file.
Sadly i haven't found any docs yet. Officially it only supports context menu for folders or drag-and-drop but i found that you can use Send To menu to open multiple files in one windows instead.