I use Microsoft Paint. It is free and already installed with Win XP and Win 7. Open the picture, click home, click resize, and shrink it by 75% or 50% (or whatever will get the job doen), then save the picture as a different name. Go to the folder where you saved the picture, right click on the picture, click properties and make sure the file size is less than 1Mb or 1,000Kb. If not, shrink it some more! You should be plenty fine to upload on this site then.
Additionally I always check the size of the picture/file before I upload to make sure it is less than 1Mb or 1,000Kb.