1. Go to File then Open and open your image
2. Choose Image Menu > Image Size
3. When the Image Size window opens, change the largest pixel dimension to 720. Make sure the "Constrain Proportions" box is checked. Click on the OK button.
4. Many images need sharpening when they are reduced in size, you may sharpen your image using the Filter menu > Sharpen > Select appropriate sharpening command and adjust to your liking.
5. Save the image: File menu > Save, Save As, or Save for the Web as a .jpg.

Images should be in the sRGB color space to look their best in a web browser. Use Photoshop's Convert to Profile function to convert your images to sRGB. Images in Adobe RGB and other spaces will not look the same in the browser as they do in Photoshop. Consult the Help menu to find this command in your version of Photoshop as it varies.

Next best bet if you have a digital camera is to use the software that came with it.

If you are still having trouble...there is Paint.

Microsoft Paint If you don't have Photoshop and you have a Windows computer chances are you have the Microsoft Paint program already installed. Try looking under going to Start > All Programs > Accessories to see if you have Paint. This program is really basic but it will perform some simple functions.
Go to the File menu then click Open and open your image. On the Image menu, click Attributes. Under Units, click Pixels. Take a look at the width and height of your image. In this case the image is 1024 x 768 and we want to reduce the 1024 side to 720 which is about 75%. Remember that and click OK.
On the Image menu, click Stretch/Skew and enter 75% for both the vertical and horizontal values so that you maintain the proportions of your image.
Now go to the File menu, click Save As and make sure Save As Type is set to JPEG. Click Save.