washingtonsblog on "How Do I Set Responsive Image Size?"

My website is Washington's Blog, and I use Twenty Ten.

I want to create responsive image sizing, where every image fills up the entire WIDTH of my content area. I don't care about height, but I want to preserve the original ratio of width-to-height.

I'm thinking of using either:

p img {
width: 100% !important;
height: auto;
}

OR

img {
display: block;
max-width: 100%;
max-height: 100%;
width: auto;
height: auto;
}

If I insert one of those CSS snippets, will it automatically resize all of my already-posted and new images to full width?

Or will it keep the old pixel settings (e.g. width="780" height="500"), and only apply the 100% width to newly-posted images with no pixel specifications?

Or will some conflict between my old posts with width/height pixel settings and the new CSS code somehow mess up the old images?

Thanks in advance!