Compression does not work very well on already compressed files.
If you want to shrink and re-save copies of the images you can use something like IrfanView's batch conversion tool(File -> Batch Conversion/Rename). You can then select the images you want to convert select a size, Check Use advanced options(for bulk resize...). Select the new size or a percentage value of the original image size. Make sure the image type is JPG and an output folder and Start Batch.
http://www.irfanview.com/
Example image.
The files are resized to 50% of the original dimensions(with resample for better quality). The output is JPG 80% quality.
The input files are 38.5 megabytes while the output files are 3.46...