My thoughts exactly. Plus the lady on the right is just sort of a hot mess.

Technically, it is impolite to tell people what to wear. However, I think the idea of using a photo on the invitation is a great way to make your point without saying a word.

Bless her heart, indeed! It was from an article called "Belles and Beer"!

The ones on the left and the middle look pretty. The one on the right looks haggard.

The background looks haggard.

#1 - looks like she's wearing a bustle or the Hubba-Hubba Hiney from Laverne & Shirley

#2 - drinking Budweiser - no further comment needed

#3 - has her skirt pulled halfway up and the "it's 1:30 am and my friend left me, can you buy me a driiiiiiiink" look on her face.

Some people will actually try to dress for the event they are going to. (Like if a wedding is outdoors, you want to try and wear something cool, not black because you look good in black.)

Other than that, without specifically telling people what to wear the closest you can do is actually say on the invitation the type of formal wear it would be.

The closest I can think of for your event is something along the lines of smart casual. (which does give space for your female guest to wear slacks if they aren't dress type).

That way you are at least giving them an expectation to try and put some effort in...at the same time not alienating folks who may not want to wear a dress, or for the men khakis.

http://en.wikipedia.org/wiki/Smart_casual - Because if its on wiki, it must be true.

Kentucky Derby style might be another way to phrase it.
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HEAD SHOP