Now that I have a DW account does this mean I have invites, too?
You might. I don't know if they automatically set up new accounts with invites anymore. If you do have any you can find them via the DW Home page. There should be a link at the top toward the right-hand side that says "Invite Someone." DW periodically grants invite codes, and I believe email notifications are sent out when that happens.

If you want to offer a bunch of people codes, you can direct them here: http://dw-codesharing.dreamwidth.org/ (They don't have to make any special requests--there are plenty of free codes for the taking in several different posts.)

Or does that only apply to paid accounts?
Nope! It's possible that they get more invite codes at a time (?), but really if anyone needs more codes, they just have to ask for them at the above-mentioned comm. They are very much about about building a community where friends bring in friends who bring in friends, etc.

And can only paid accounts add a graphic to their journal header? I couldn't quite figure out how to get a nice pic of Sam and Dean on mine. :-)

Now this I was unsure about (I have a free account and have never tried to customize my layout beyond the color LOL).

This page list the breakdown of account features between Free/Paid/Premium levels: http://www.dreamwidth.org/support/faqbrowse?faqid=4

It mentions free accounts have "limited" ability to create custom styles, so that may be one of the limits--however, I didn't see that spelled out anywhere.

If you are trying to copy a style from LJ over here to DW this post may be helpful: http://www.dreamwidth.org/support/faqbrowse?faqid=171

Also, here is a link to the tutorials tag at dw_styles: http://dw-styles.dreamwidth.org/tag/tutorial

Sorry, I can't be more help in this area--the customizing layout stuff tends to go over my head. :D