Adding White Space To A Thumbnail?

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 are not 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 ratio? Or can someone think of a good way to do this with CSS and HTML, OR even with javascript (keeping in mind that my images come inside a repeater bound to data in the db.

If you want to see any of my code or the thumbnail creating class just ask.