@Ronel: It cannot be done perfect like that as your image size and the percentage of width need to match. If you want like that then you might need to consider using plugin. I recommend you the plugin like “Jetpack” where you have module like “Tiled Galleries”.
For this gallery, either you have make it aligned left, center or right. So, best option will be to align center. For that you can add the following css in “Appearance => Theme Options => Custom CSS” box:
.gallery .gallery-icon img {
display: block;
margin: 0 auto;
}