Magento Configurable Image Swticher adds feature on product page for configurable products. When customer select option of configurable product on product page, main image changes to the image from selected simple product. If your associated simple product doesn't have any image uploaded, it may change to the default image placeholder or didn't change image (it may be configured in extension's admin settings).
Also extension support integration with custom zoom modules with different image size from base template. If you have custom template with custom zoom you can order installation support with the extension. With default seeting it support magento native zoom function. Make sure that you have compilation disabled before installing extension. System > Tools > Compilation page and click on Disable button. After Installation you can enable compilation back.