for responsive images here's what I do.
<img src="image.jpg" width="600" style="display:block;width:100%;max-width:600px" />
Modern email clients will look at the css stylings for scaling and ignore attribute values. Older clients like outlook and lotus notes (except Lotus Notes 8) will look at the attribute value.
The alternative here would be to target specific images using classes instead of using a catch-all.
<img src="image.jpg" width="600" class="responsive" />
then in your css you could put:
All you need to do is wrap your Alternative text container with a conditional comment like this:
<!--[if !mso 9]><!--><table class="alt-text">...</table><!--[endif]-->
One thing to note is if you do this, nothing will display in outlook 07, 10, and 13 until images are turned on. Personally I'll take duplicate copy over nothing being displayed with images off.
Some may not agree with this approach but I negate the scaling effects of gmail app with this snippet of code at the bottom of my emails (before the closing body tag)
<img src="spacer.gif" height="1" class="hide-md" style="display:block;min-width:500px;">
This spacer image will stop gmail from auto-scaling your email to oblivion.