Add this code to the Custom CSS field on the Style tab of your Portfolio widget’s settings:
.global-styles, .eapp-portfolio-project-viewer-component .eapp-portfolio-picture-component {
height: auto !important;
}
.global-styles, .eapp-portfolio-project-viewer-component .eapp-portfolio-picture-img {
max-width: 50% !important;
margin: 0 auto;
}