We never see a photograph of Rebecca

I've always thought we don't see a picture of Rebecca becuase that is part of the tension that exists between the subjective views of beauty alluded to and the if you will objective state or level of beauty of Joan Fontaine herself. Such relationship was not intended to be clear, and imo the viewer is supposed to see some conflict between the social construct of how others perceived Rebecca's beauty as part of her overall character and social presentation, compared to the second Mrs. de Winter's lower social standing, simpler manner of dress, makeup and hairstyle, and of course her shy and withdrawing manner.

While for the most part I give great credit to Ms. Fontaine for having pulled that all off, a part of me finds the whole notion of her having a plain appearance to be rather silly. She was in fact one of Hollywood's great beauties, and "objectively" was arguably a poor choice purely on grounds of a surface distinction between her and even the supposed great beauty of Rebecca Hentridge. (This was taken even farther with Ms. Fontaine in her later effort in Jane Eyre, but at least there the filmmakers used a variety of lighting, makeup, dress and other stylings to play down her natural beauty.)

But as the film progresses I think we begin to understand that while Rebecca no doubt was beautiful, much of hte putative distance between her lofty place and that of the character as portrayed by Fontaine is slowly understood to be in large part based on social expectations.

There is a specific part of the film where we see this at work. When Bea asks the second Mrs. de Winter about her hairstyle, and if she can't do something about making it better, she later asks Maxim what he thinks about her hair. On one level he is dismissive of the question, but I think not in the way husbands and lovers know to be careful with too much honesty when invited by their wives or girlfriends to consider giving a negative opinion about their appearance. I think it is more than that. I think he knows that while Rebecca was objectively beautiful, Maxim came to distrust the results of the efforts undertaken to maximize the effect of her beauty, the clothes, the time spent on her appearance, and all that sort of thing.

Joan Fontaine of course could and did previously portray characters that used a great deal of makeup and other fancy accoutrements of beauty, like her character in The Women. But here she was called upon to play a woman of a more natural sort of beauty, and no doubt her ability to do so was an essential part of why she was given the role.

But of course it is not only the viewer's understanding that is significant here, but also of Fontaine's character. The notion that Rebecca was more beautiful than her was part of what was feeding into her insecurity. Left to wonder what Rebecca actually looked like, we can imagine her imagining that in fact it was a great distance between them. But at some point we return to the objective reality of her own beauty, however much she downplays it with her acting and unpretentious dress and hairstyles. And of course there is also the obvious question why Maxim would choose a second wife who would be much less attractive than his first wife. The answer is that in fact he did not, and by the end we realize it is not only the absence of Rebecca's narcissism that makes his second wife the more attractive. It is that she is in fact very attractive herself.