In admin go to the System > Manage Images to resize all images of your system.

Here You can see a grid having data like Image Id, Path, Resized Status and image Action.

Sync Images:

• Click this button to Synchronize all the images.
• At the initial stage the grid will be blank.
• After clicking on the Sync Images button images will be displayed in the grid.
• By clicking this button all the new images will be loaded.
• So you can see all the images of the store loaded in the grid

Resize Images: Click this button to resize all the images in the grid automatically.

Mass Actions

• Delete: Select the images you don’t want to resize.
• Change Status
  • Required: You can choose multiple or all images which need to be required.
  • Done: Select multiple or all images which do not need to be resized and already resized.
  • Skip: Set skip status by selecting the images which you want to skip in the resize process.
• Resize Selected Images:
  • Select the images you want to resize.
  • From selected images only images having required status will be updated and resized.