In the "File" menu - choose "Batch Conversion/Rename".

 

Select the following options:
- Work as: "Batch conversion"
- Output format jpg, gif or png.
- On the right side, find the directory where your images are and click "Add all"

Check "Use advanced option" and click on the button "Advanced".

- Check RESIZE and choose "Set new size as percentage of original" (maybe 10%, it depends on the original file size)
Click OK to close the advanced dialog.

- Choose "Output directory for result files" (should be different directory than where the original files are)
- Click on "Start Batch"