bimgConv (Batch Image Converter) helps you with that, changing canvas size of pictures. It also allows to resize pictures, all drag & drop simple.
BatchImageConverter runs anywhere and best of all it does not require any installation. Just download, unzip and youÃ¢â‚¬â„¢re ready to go.
Works on Windows (Ã¢â‚¬â„¢98 and above), Mac (Universal Binary), Linux (GTK 2.x required).
Download: bimgConv BatchImageConverter