Contact Form on WordPress Sites?

How does one go about building a page in wordpress that contains a contact form full of custom fields, boxes, inputs, etc. completely determined by the author of the site. It seems odd that wordpress currently lacks this feature, but knowing the wordpress community, it’s unlikely that we must do without.

How do I change/modify the_post_thumbnail(); html output?

I’m working on building out a custom theme and have been struggling with this one for a bit. I am trying to modify the HTML output of the_post_thumbnail(); function. I need to do this because I am trying to support retina images on my site and would rather bake the functionality into my theme than load a plugin on the backend.