Hi there, that’s not going to be easy unfortunately, as some of the poster images aren’t to scale as they’ve got a white background behind them saved into the image.

You could try something like

.rf_latest_posts_widget .postimage-container a {
background: url(“images/glass_tiny.png”) no-repeat scroll 0 0 / contain #ffffff;
border: 4px solid #f0f0f0;
display: inline-block;
height: 100% !important;
overflow: hidden;
width: 100% !important;

But the layout might look a bit weird – http://screencast.com/t/rkHUrBluu

WordPress uses 150 x 150 for thumbnails as standard.