Go to https://pixlr.com/x/
Click [OPEN IMAGE] and select the file from your computer,
Click the “Arrange” icon
Click [RESIZE IMAGE]
Enter the new width or height in pixels, if “Constrain Proportions” is selected then changing one will automatically change the other. Once you have the correct size click [Apply]
Your image has now been resized, click [Save] to open the download dialogue box.
If saving for the web, reduce the quality using the slider as much as you can without the image distorting (The medium preset works well) and then hit download.
The image will download to your default download location.