If most of your guests are local or if the kitty party is an easy family get-together, the invitations can be super-simple. Try electronic invites through Evite or Facebook. You can create an event and invite people without sending out paper invitations (for free). You can cut the cardstock into the shape of photo frame and decorate its boundaries with colors and flowers and paste the picture of couple on the frame invitation card. Write the invitation details with the following wordings.
JOIN US FOR THE CELEBRTION
OF 50 YEARS OF TOGETHERNESS
ON 27TH MAY AT ROYAL COTTAGE
FOR GOLDEN JUBILEE THEME KITTY PARTY.