Help me with this CSS please!

I’m using a lot of themes because I can’t add a css line at this:

http://img163.imageshack.us/img163/467/giqb.png

I’ve tried:

a img

img

even a cascade of classes, and it doesn’t work, I cant apply anything

Oh, And I’ve just could apply the css, but it applies to all the div, and if I have a border on the image and I apply a padding, the padding goes separate within the border.
That was with this css code:

.entry-content a img, #attachment-nav a img {
padding-left:30px;
}