Could use some advice
I have written briefly on here about my situation with BM. She has made it very clear that she does not like me and does not like that I am in her daughter's life. However, she and my DH share 50/50 custody. We also live in the same small town and my DD is involved in the same activities as SD. There is no getting away from each other. My SD and I are very close, we have a great relationship.
BM bought SD a cell phone for Christmas and gave it to her last week before SD came to be with us since we would have Christmas first (she does this every year, we just ignore it). She then proceeded to text and call SD all weekend while we were visiting with family and celebrating Christmas. My DH finally took the phone away from her. SD went home Christmas at noon and by 12:45 my DH was receiving text messages from BM complaining about me texting her daughter. She told him that I have no right to tell her daughter that I love her. I am not her mother. She even went through SD pictures and deleted a picture that SD took of her and I. SD is 10 and is old enough to know what's going on. She was very upset when she came back this morning about how her mom kept complaining about me all day Christmas and how her mom deleted the pic (she went in her pictures to send it to me this morning, that's when she saw it was missing).
We deal with this sort of stuff regularly. BM wants me to do all the work: run SD to practices, do homework, take care of her but says that I need to "know my place". I try to ignore all of this. I bite my tongue when SD comes crying to me and when other parents tell me how BM has been badmouthing me. It is getting harder and harder to keep my mouth shut and I could really use some tips/pointers/advice anything to help me get through this without making it worse 
Re: Could use some advice
I think this is great advice. Keep being a great role model to your SD. I was on the receiving end of all that for many years with my SS's BM. Only in the past few months did she apologize to me for being so horrible. I have been in SS's life 8 years now, he is 13. She said she had a very hard time having another woman playing a motherly role to her son. To some extent I can understand that, but it did get very bad for awhile and was upsetting SS. I hope that the bm in your situation can realize what she is doing is not worth it.
Also, I always kept my cool around SS, continued to never bad mouth his mother in front of him, or even when he was in the house. I just couldnt feed into her games.
Good luck, I know its tough.
I understand what you're saying but there is more to it than that. My SD does still believe in Santa (or at least hasn't admitted it out loud yet). The reason it bothered us is because last year, SD wanted an American Girl doll. That was the big gift. BM kept asking my DH if we were getting it because she didn't want to get the same thing (even though she has insisted that all belongings be kept separate). My DH told her that he didn't think we were because we got her a Nintendo DS. Christmas Eve, my SD came to our house and had just opened 1 gift (which her mom never used to allow her to do) and it was a Nintendo DS. She intentionally does this stuff...she got her a cell phone bc SD told her that was the big gift she was asking for at our house.
So don't tell her what you're getting. Subterfuge, circumvent or do whatever. If you can, buy two big ticket items you know she wants and give her the one mom doesn't (and return the one she did buy her.) Petty but if it's what needs to be done to keep the peace...
(a biomom who doesn't play those games and hates headgames with children.)
If you want to text your SD "I love you", do it. Let BM continue to play her petty juvenile games. It will bite her in the ass later. As for the pictures, that's very unfortunate but at least you have lots of other photos of you 2 together. I know it's hard when the little ones get upset by this behavior, it breaks my heart sometimes when my bonus daughter comes to me sad and confused by her mother's comments and actions. But as long as you reinforce to her that you love her and care about her, she'll come out of this ok.
If its any consolation, my bonus daughter's mother pulls the same crap and bad mouths me to every teacher, parent, coach, etc. And guess what? As soon as those people saw me interact with my bonus daughter and saw how much she clearly loves me, they all began ignoring her BM. Actions speak louder than words, and there are not enough negative words to dispute how involved you are with your SD and how much you care about her. So keep doing what you're doing. You are obviously not the one with the problem, she is.
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I have contact with my oldest bonus daughter through email, texting and her cell phone. I don't call the house unless there is a giant emergency, because I know their mother would go crazy and I don't want to make their lives difficult.
I would make something fun out of it. Create a crazy phrase that means "I love you" but that no one else would understand. Something like "Eat your peas" or "Shut the Door" (a misreading of the french phrase "Je t'adore").