As an artist and wannabe game designer myself, I still can’t help but agree. A toggle would be great, though it would be even better if there were two options: to only hide character portraits, not maps or other kinds of illustration (a visual guide for a puzzle, etc) and, optionally, to hide everything and swap all illustrations with descriptive text (which is something a lot of designers are used to do, for example, for accessibility in websites with a text-to-speech function: the tool describes the images within the page as well).
If I may be so bold as to give a suggestion, I think that a lot of it this “uncanny valley” feeling could be circumvented if the character illustrations still left some or most things to the imagination. That is, if the illustrations themselves were more stylized than realistic. One simple way to achieve that for most content creators on a budget is by giving a silhouette of the character, not a full illustration. But if there’s an artist involved, there are sooo many ways this could work… If the story happens in the old Egypt, for example, the characters could be shown as illustrations in hieroglyphic style, which gives a general sense of how the character looks but still leaves a lot of for the player imagination to play with. One excellent example of the use of this stylized artistic direction are the tarot cards in Dragon Age Inquisition.
I think it’s important to consider if the illustrations are there with the purpose of enhancing the sense of immersion and improve one’s experience/gameplay. If the art is there exclusively to “look pretty”, then there should be none at all imo. It is a problem many hyperealistic-focused games have nowadays… There’s a saying where I’m from that roughly translates as: “If one one only sees the face, they can’t see the heart”, well, “if one only sees realism, they can’t see art direction”. Haha. 
And if there’s a concern about the game becoming too slow on mobile, I recommend to vectorize your art instead of using .png/.jpg images. Vectorized artworks usually come most commonly in *.eps and *.svg formats (and yes, websites use those all the time, especially in logos and animations, and basically all text fonts are vector data). Because a vector uses a mathematical formulae to drawn shapes and lines, contrary to bitmap tech that creates images from a lengthy list of pixel per pixel data (this pixel is this color, this pixel is this color, next pixel is that color and it goes on and on forever until it shapes the whole thing…), they have a much smaller file size and are processed much faster by any machine. Also, vectors can be scaled to infinity without ever losing image quality/looking blurry, because you are basically just changing the values in a math equation. There’s no “pixel stretching” since it doesn’t drawn things per pixel to begin with.
The process of transforming an bitmap image into a vector image is called vectorizing. It can be done with both free tools and paid ones… Or one can drawn an illustration as a vector from the beginning, though that may require some additional study as it is a bit different than drawing the usual digital art.
