How do you change the image size of a picture on the computer?

i would like to change the size of a picture on the computer so that the size is 500k or smaller but im not sure how to do that.
i got the picture from my digital camera.

Use any picture program -I prefere the freeware Irfanview- to open your pic. Change the "dpi" settings to 72, reduce width and height, save as .gif, .jpg or .png. If you want to use it as an avatar for YA, save as .jpg and reduce size to 48x48 pixels - this is the most common size for YA avatars.

http://www.picasa.com