Xnview is an interesting way to batch process thousands of files, because you can also apply some minor but effective filtering in the batch process. When I wanted to process over 1 million files, it was way faster than any of the photo programs scripting or batching methods, and the design of the filter elements work the way a person would want them to. Like the interesting conbright and gamasat, good settings for simple sharpening , and fast filter operations all done in cpu. You could also easily set the quality of the jpeg compression/destruction for the output.
I recommend not because it is free, but because it blew away a popular photo program speed wise by more than 10 times.
It would take some time to learn.
To get you started generally:
In the tools menu select Batch processing, select the files or whole folders.
Avoid the scripts as you are not likely to need it for simple stuff.
Set the directory (preferably a New one, not overwriting)
Set the Options for the file format (jpeg) be sure to set the options before starting the process.
Go to the transformations tab, and select a resize, hit Add and adjust the resize values.
If your resizing down it can be helpfull to toss in a bit of sharpening, also in the transforms tab.
Hit the GO button after you have set everything.
Being a bit lazy I will get the batch going, the process is so fast, that I can see how it is working out, and stop it and delete the finals, and make a few more adjustments and start it again.
Because this is mostly GUI based, you can test before hand the same actions, and see easily what will occur, then batch the same action to a large set of files.
That is what I use when there is not Re-Touching of the photos, the photos are all mostly the same size and format, and/or the same changes would be useful to all of the pictures, and I just want to process them as best as can be done in a auto process.
Anytime your trying to recrunch high loss format like Jpeg, into just makeing its file size some smaller, if you can choose to resize the file clean it up some , and save it out a at Smaller resolution size with less heavy compression, that will be better than trying to just recompress it and store it with even more lossy compression.
Nothing will beat the most original photos (even highly compressed) for archiving the originals or preserving the originals. Unless the information is discardable, a person would want to think about keeping the Originals, Writing them to a couple of CDs or storing them on a back-up hard drive.
When at work processing peoples photos, having the originals to work with is gold, all the crunched and made for web and smaller versions of them might make better Output, but if they are needed for a different process, or project only the originals (as badly as they may be crunched) are as original as we can get.