Yeah, I saw that picture, too. Are you sure it was with Leanna? This was when Phil was still trying to hide Kat's appearance (there was also the "behind the back Christmas tree picture").
I don't wanna go too much into something so stupid, and I don't think it can be confirmed either way.
Going by these "stalker" pictures that were taken of Phil and Leanna at the mall (some think that's not even Phil), you can gauge their height in relation to each other.
At 6:47 of his "Holiday 2017 House Decoration Walkthrough Vlog", Phil, after shrewdly turning a framed photo to the wall, leans forward to switch off the fireplace, revealing a grainy glimpse of the hidden picture.
You can make out two people in the portrait. The figure on the right with the gout-ee is unmistakably Phil. The figure on the left is wearing a Santa hat. Considering Kat had just visited, it'd make sense that one of them would be wearing a festive hat for a photo recently taken together.
Kat and Leanna's comparative height seems eerily close.
Despite the similarity, I do think it makes more sense for it to be Kat.
(@Prince Lotor if you were just joking about it being Leanna, then this was doubly stupid
).