Magento Image Compression

If you would like to get control on the quality of your Magento product images then you need to follow a few steps:

STEP 1.
Copy ‘/lib/Varien/Image/Adapter/Gd2.php’ to ‘/app/code/local/Varien/Image/Adapter/Gd2.php’. You will need to create any directories that do not already exist before copying the file. Edit the copied file, not the original. This will prevent the file from being overwritten by upgrades.

STEP 2.
Find this code on line 80:

call_user_func($this->_getCallback('output'), $this->_imageHandler, $fileName); 

And replace it with this code:

if ($this->_fileType === IMAGETYPE_JPEG) {
            call_user_func($this->_getCallback('output'), $this->_imageHandler, $fileName, 99);
        } else {
            call_user_func($this->_getCallback('output'), $this->_imageHandler, $fileName);
        }

Remember that if you don’t copy the file to your local folder it can be overwritten every time you upgrade, even if there are no changes to the file.