Adding white space to a thumbnail

Hi everyone,
In my website, users have the ability to upload images to my website. When the image is uploaded it will automatically have a smaller thumbnail created by my website. The thumbnails arent displaying correctly in my repeater though (and by correctly I mean Im not sure how to display them).
 Each image sits inside a square div element which is 145 x 145 pixels. Some of the images are 145 wide with a relative height and some are 145 high with a relative width. When my page is displayed the wider images sit at the bottom of my 145 x 145 div, whereas the higher image sit at the bottom and to the left. I would prefer them to all be centered.
 I cant find a way to do this is in CSS becuase the images are retrieved dynamically so I cant assign a height beforehand. Is there a way to add white space to my image during the thumbnail creation process so that all images are 145 x 145 but still maintain the aspect r ...