Centering Images

Basic question about centering the thumbnails in my sidebar. I have the Top 10 plugin active in my right sidebar. For the most part, I’m trying to keep it clean and basic with just the thumbnails showing down the right side.

Currently, the default aligns everything to the left but I would prefer they be centered since I am not using any text nor titles. I don’t use the Timthumb to display the images rather just having them pulled from my posts.

I’ve tried to find where I might change the CSS to something like

img.center {
display: block;
margin-left: auto;
margin-right: auto;
}

but can’t seem to locate it. Any suggestions would be greatly appreciated. My site is https://partyfavorz.com