Find the section in your theme's CSS that applies to your sidebar (could be a #sidebar or .sidebar id/class section) and add the following:
Code:
#sidebar img {
padding: 2px;
}
This is technically a separate block on it's own (because of the 'img' after the #sidebar), but you want to find the section corresponding to your sidebar so you know which id/class you're assigning this value to, otherwise it will apply to all the images on your site.
If you don't care, or want to have this padding apply to all the images of your site, then simply use the following somewhere in your CSS file:
Code:
img {
padding: 2px;
}
You can specify the padding value to be however large or small you need/want.