It was love at first sight...for Tomoyo, at least. But she knows that Sakura doesn't feel the same way about her, and so she's never said anything, and probably never will. Tomoyo seems willing to just be Sakura's best friend. She is even truly supportive of Sakura's crush on Yukito, and she actually tries to help Syaoran when he realizes his feelings for Sakura. Even if she didn't flat out admit that she loves Sakura (which is what she is saying in the picture), it would be completely obvious. She follows her around with a video camera, is constantly talking about how perfect she is, and falls over in swoons when Sakura does something particularly cute. Not to mention that she spends a lot of her time watching and re-watching the movies she's made of Sakura. Tomoyo has even kept the first gift that Sakura ever gave her. The first day they met, Sakura gave her a tiny eraser, and she has kept it in a safe place ever since then. It is her most precious thing. Now, if that isn't sweet, I don't know what is.
Tomoyo seems to be following a family pattern. Her mother, Sonomi, had been in love with Sakura's mother when they were young. Sonomi let her grow her hair out because it looks like Nadesiko's hair. Tomoyo keeps that eraser in the same box where Sonomi keeps Nadesiko's wedding bouquet. But Sonomi couldn't be happy for Nadesiko when she fell in love with Fujitaka. Luckily, Tomoyo is much more understanding, and only wishes for Sakura's happiness.