Assuming the website doesn't do it automatically, you need to use an image editing program (pretty much anything will do it; IrfanView is free) and select 'resize' or 'image size'. Then 'save as' as a new file. Don't do 'save' or you'll lose the full res version.
If you have microsoft office on it should come with picture manager. You can resize in many ways with that , from pixel size to % ratio of the original
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