You don't want to set a max-height. this will have ill desired effect to your width...
.ow_photo_attachment_stage, .ow_photo_attachment_pic.ow_attachment_preload {
background-color: transparent;
background-size: 100% auto;
border: medium none;
max-width: 120px;
width: 100%}
.ow_photo_attachment_pic.loading {
background-size: 16px 16px !important}
Note: set max-width to fit your need, mine per example below is set at 200px,...
.ow_newsfeed_item_picture img {
height: auto;
max-width: 200px !important}
Note: the extra provided .ow_photo_attachment_stage, .ow_photo_attachment_pic.ow_attachment_preload -AND- .ow_photo_attachment_pic.loading I provided have to do with the upload process/Look of the thumbnail, this is NOT needed to solve your issue at hand, I blame this need to provide extra's on a mental condition lol, or just maybe a stickler of sort for details,,, No-matter, Enjoy..