I consider myself quite a visual person. My mother and step-father are both visual artists (my step-father sadly passed in 2010). I also used to draw a lot when I was younger (I think I went to my first nude drawing class when I was about ELEVEN!). This included illustrating a lot of my own stories.
Now when I write, I keep a folder on my computer which has sub-folders full of photos of the settings and characters in my manuscript, most of which I source online. The physical appearance of the character of Lillie in WHEN THE WORLD WAS FLAT (AND WE WERE IN LOVE) was based on actress Felicity Jones (yes, I loved her WAY before Rogue One). The physical appearance of her love interest (or should I say, SOUL MATE) was based on Jensen Ackles (noting Tom is only seventeen and Jensen is thirty-something).
I find having a visual reference can REALLY help to ensure consistency with your physical descriptions throughout your manuscript. For example, if you describe a character as being as thin as a reed early in your story, you wouldn’t want to describe them as hulking over another character later on. OR, if you describe them as fair skinned with freckles, you’ll want to remember that when you have them standing out in the middle of the day without sunscreen (one of my characters in IN THE BEGINNING THERE WAS US gets badly sunburned due to this very reason).
Without stalking TOO much (i.e. don’t take your own photos unless you A: know the person and B: have asked if you can take their photo and stick them in your book!), try and get a range of photos with a number of expressions. This will be handy for those scenes when you need more than just a smirk or a frown. Are they an ugly crier? Does a vein bulge in their forehead when they yell? For this reason actors/actresses can make the best character models (and, of course, you can let the casting agent know who should play the lead when you get your movie deal…).