Truly, the phrase "garden tea party" should indicate what to wear. If there's not an exact title, you could somehow work in The Stepford Look Garden Tea Party or something simular. But actually telling someone how to dress (with the exemption of casual, business casual, semi-formal, and formal) is a tacky way of telling your friends that they don't know how to dress - even if you
do think that they don't know how to dress! Of course, when the guests RS their VP, be sure to gush that you just can't
wait to wear the darling sundress and hat you found!
If you're sending invitations in the mail, take a picture of the hostess and one or two friends in the appropriate attire first, then use that with a "Come Have Tea With Us!" on the cover.
I also like AGDee's ideal of subliminal messages; having muted photos in the background. When I first saw this thread, I thought of the Designing Women photo:
Or
Or
There's also a magazine called Tea Time. It should have the appropriate pictures. There's some great ideas in there, from recipes to mixing tea cups to cookie cutters to have decorated like tea dresses.
Good luck!