Here's a couple of free tools to resize all your images at once or individually. This will save time and make them viewable for your online store.

  Option 1 (Bluefive) - Download the free programme to your desktop.
I prefer this tool as it gives more flexibility and is a little easier to understand.

 Option 2 (Visualizer Photo Resize) -  Download the free programme to your desktop.

Once it's set up, place all your images into the one folder and use the programme to resize them. It will place a new folder where you chose with all the smaller images ready for uploading to your store.