Download the free program by clicking here and choosing to Save the file to your Desktop.  

Go to your desktop and run the installation file - iview390.exe 

Follow the installation steps, accepting all of the default options (unless you would like to change them - just be sure you know what you are changing and why). 

The installation should put an icon on your desktop called Irfan View. Run this program. 

Click File -> Open and browse to where your picture file is located. 

This is the file we opened. Please note the size, you will see that it will be smaller after we are finished resizing the image.

Click Image -> Resize/Resample 
(Also please note that you may rotate or flip the image. IrfanView is an extremely powerful easy-to-use tool. It has built-in preview and slide-show options also. You may view more documentation, download plug-ins (nice additions), and more from their official site - http://www.irfanview.com). 

Once you select to resize your image, you are presented with the following options. There are 3 main ways to resize your image: 
1. Set the new size by entering in the size you want the image to be (make sure Preserve Aspect Ratio is checked when using this option). 
2. Click the "Set new size as a percentage" button. This is normally the best way to go unless your images are standard image sizes (sizes listed beneath the Half & Double buttons).  
3. Resize your image by clicking the "Half" button or selecting one of the image sizes. These are standard photograph image sizes and work extremely well if you are working with images from a Photo CD.  
Note: If you select to resize as a percentage or to set a new size, make sure the "Preserve Aspect Ratio" option is checked. This will allow you to enter in one of the resizing paramaters and IrfanView will automatically set the second variable for you.

We are going to make our image 50% smaller. We have set the resize percentage to be 50%. Please note the new size that is listed now. This is very handy if your image must be under a certain width or height.

Click OK. 
Now we have our resized image. You may compare this image with the one above to see the difference in size.  
 

Now we need to save our resized image. You may simply save it, but then you will be overwriting your original image. In many cases you do not want to do this. To preserve the original file, we are going to use the Save-As option. 
Click File -> Save As 
 

Select the location where you wish to save your resized file.  
 

Now you need to select the type of file you would like to save your image as. 
This is another really nice feature that IrfanView offers. You may convert images from one file format to another. Most web browsers are only capable of viewing images that are .gif or .jpg. 
   .gif images are normally cartoonish (drawn on a computer) or mainly text. 
   .jpg images are normally pictures / photographs.  

We are going to save our images as a .jpg (although the best option to save this file as would be a .gif. We are not doing that because this image is part of a series of images that are .jpg). 
Click the drop-down arrow beside the Save as Type option. 
Select the type of file you would like to save your image as.  
 

If you select a .jpg or a .gif, you will get another option box. For .jpg images, you can select the quality you wish to save the image as. For the web, most .jpg files are saved at a quality of 60-80. You may save as a Progressive .jpg if you wish. 
For .gif files, you may save interlaced if you wish.  
 

 
That's it! You have now opened a file, resized it, and saved it as another format. 
Best of luck with your images!