How to center widget?

I will need more context to make a suggestion. What theme are you using? Can you provide a link to your site or a screenshot of your site? Do the images need tonsit in the middle of a sidebar or footer?

You will need to create some custom CSS for your site. Do you know how to add CSS to your theme?

Try one of the following:

.instagram-pics li {
    text-align: center;
}

or

.instagram-pics img {
    display: inline-block;
    margin: 0 auto;
}