You don't even need to open the picture file, or have any computer skills at all to use this little gem. Very useful!
Go here to download and install the program, appropriately named Image Resizer:
http://imageresizer.codeplex.com/
Read all about it, then go to Downloads and choose the 2.1 version for your OS, 32-bit or 64-bit. Choose 32-bit if you´re unsure what to pick, as it works also with 64-bit systems, but not vice versa. (You can also get the Preview 3 version if you feel a bit adventurous).
Once installed, you won't se much of the program at all for most of the time. No program folder to navigate, no program icon to click to start the program...
Image Resizes always resides in the background and steps forward automatically to do its thing only when needed:
Resizes a picture to your choosen pick and taste; usually smaller than a huge digital camera file.
Just put the cursor on the picture file you want to resize, right klick and choose "Resize Picture" from the menu.
You get multiple choice options for size, and also custom for specific formats of your own choice, both larger and smaller than the program pre sets.

Click OK and a new and much smaller version of the picture shows up in the same folder as the original file.
The file name will also stay the same as the original, only with an added (Custom) within brackets as a marker that the pic has been processed.
You can can choose to resize just a single picture file at a time, or several pictures in a folder by highlighting them, or even a whole folder containing hundreds of pictures. The latter may take a while, though. Time for a stroll or a coffee break.
Happy resizing!
Oh, BTW. This program is PC Windows only, sorry.
I haven't yet found anything like it for the Mac, other than the scripted feature in Photoshop.
But I've found this little program so useful that I happily transfer picture files back an forth between my Macs and my PC just to resize them, as I've found that the resulting resized files from this little program is even smaller, file wise, than even optimized Photoshop files. Very impressing!