on a Windows pc just use the standard picture manager.
Double click the image and unless you've got jpg's associated with something else, they will open with picture manager. Click edit on the top tool ribbon and then resize( on the right hand side)
There's an option to resize to "web page -large" or "web page small" I used 'large' but small would probably been enough. Select ok
That's all from memory but I'll double check tomorrow night. It's nearly midnight here and I'm off to bed