If you wish to alter the dimensions of the catalog images in your Magento store, then the Catalog Images Tool is quite handy for this purpose. It lets you override the default size and proportion of any image in your catalog layout. You can do this by simply specifying the new dimensions of the image in pixels. This tutorial will assist you in resizing catalog images and restoring them to default size in easy steps.
Magento by default adds borders for images which are smaller than the bucket size. But you can disallow Magento to do so and change the default settings. Clear the Add the white borders to the images that that do not match the container size checkbox, if you don’t want to save the borders to each image.
b) Small Image
c) Base Image
Select one of the image types to set to a custom size and the other two types will be resized automatically.
Resizing catalog images for a single theme in Magento applies for all other themes as well. But in order to render the images correctly across all themes, you need to update any custom CSS you made.