If you don't have Photoshop, you can use XnView
Open the picture within it, click Image, then Resize. Make sure 'Keep Ratio' is checked, and that 'Resample' is unchecked. Under 'Print Size', 'Units' should be set to 300 and the drop down menu beside it to 'pixels/inch'. Click 'Ok'. Now just a matter of saving and repeating.
Or, you could use this Batch Resizer
When you download the file, rename it to "PhotoResizeD300Q100.exe". Now, don't run the file. Highlight all the photos you want to convert, then click and drag that selection over the icon of the downloaded file and then release your mouse. An automated terminal window will open up and list off the conversions. The converted photos should now be in the same folder of the originals.