It also occurred to me that it might have meant "take a photograph of M while on vacation", which is much more sinister.
As I recall, there are references to Annasmom in the BFH where Waters lists her name as the initial "M". As I recall the timeline of events, the "vacation" in 1971 would have been near the end or immediately following the conclusion of divorce proceedings. Waters' writings at the time were filled with venomous remarks concerning Annasmom (which puts this picture comment in an even more sinister light).
Recall that two years earlier (at the start of the divorce process), Waters had sent the "Rommel Letters", attempting to learn the exact dosage of cyanide that General Rommel and Herman Goerring had used to commit suicide. Annasmom and I have differing interpetations of this: she suspects that he may have been considering suicide at that point, but I wonder if he may have been considering something worse - murdering Annasmom. I cannot see Brody "approving" of Waters killing himself, since suicide usually invalidates any life insurance policies and Waters would not have acted without Brody's permission. So if the cyanide was not to be used to kill himself, who was it for? It seems logical that it could have been meant for the person who Waters felt was responsible for anything that had gone wrong for him and had rejected Brody as the man "more gifted as Jesus" - Annasmom. My suspicion is that he either did not receive the information that he sought, or that he lost his nerve, so the two Georges moved on to another plan to exact revenge - to abduct Anna.
We already have concluded that the two Georges always seemed to convince themselves in the justification of their actions - that, in their minds, they were agents of good. Toss into this mix the bad feelings (fueled by the rantings of Brody) that Waters felt toward Annasmom and you can easily see that Waters and Brody might feel justified in removing Anna from what they saw as a bad situation of being raised by an awful person. This could be a motive.
The will leaving Anna $1 shows that they also were concerned that Anna was a threat to Brody receiving any estate from Waters if he passed away. The BFH contains a letter to the U.S. State Department asking that Brody (and
only Brody) be notified if Waters were to die while overseas, specifically asking that no family be notified. The insurance policies were drawn up with Brody as the "irrevocable beneficiary", meaning that as long as Brody was alive, nobody (including Waters) could change the beneficiary. All of these point to the fact that Brody (and, by default, Waters) was extremely paranoid that Anna (as Waters' sole legal heir) would somehow prevent Brody from getting "his" money. This would also provide a motive to make Anna "disappear".