I'm working on a site at the moment (not live yet) for an artist showing his paintings in a grid display and it sounds like I came up against the same issue.
Here, the original canvases are different sizes so the aspect ratio of the images vary; some are landscape and some are portrait
The best solution for me was to specify the featured image size as a square (use 385px × 385px as an example) with no cropping so the landscape one come out at 385px wide with a different heights and the portrait ones 255px high with different widths.
As each image is within a link, we can use the link as a containing element specified as a 385px square with the image sittin inthe centre of the square at its original aspect ratio with either a shaded background showing top & bottom or a shaded background showing left & right
The css I'm using for this is as below, perhaps something similar adjusted for your site would help?
.artwork-featured-image a {
background: #ecede5;
border: 0;
float: left;
margin: 0 0 12px 0;
padding: 0;
height: 385px;
width: 385px;
position: relative;
margin-left: auto;
margin-right: auto;
overflow: hidden;
}
.artwork-featured-image a img {
position: absolute;
margin: auto;
top: 0;
left: 0;
right: 0;
bottom: 0;
}