Here's a few free tools to resize all your images at once or individually. This will save time and make them viewable for your online store.
and a recent update for Mac compatible options.
http://downsize.en.softonic.com/mac
http://smallimage.en.softonic.com/mac
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.
