I know it's a bit late but still...
IrfanView's Batch Conversion/Rename [B] will do the trick. Select the folder where all your albums are stored and be sure to check "Include subdirectories". Click "Add all", adjust the "Output format" options for JPEG and set "Advanced options" to resize the images to 300x300 px. You'll also probably want to apply Sharpen filter if you're scaling down images.
