most pic viewers have an option.. usually called "Resize/Resample" or something close to that. when you click it, it will show you the dimentions of your pic size... height & width, change the larger of the two numbers to 150.
This will make your pic the right size.
Then choose "File" then "save as" and save the file as either a Gif or a Jpg.. and thats it! Post the pic to a URL somewhere or simply upload it from your PC
well.... thats how I do it anyway