For all we know EK could've had some idea for this epilogue that got pushed back, but the pictures were already made at this point and got accidentally included.That theory Seems like far fetched to me, Chapter 13 is going to be mostly Lena's POV with smaller Ian pov at the end of the chapter, which I reckon will take atleast more thn half the year if we're lucky. I don't think Eva has planned that far ahead and commissioned art for it as well.
Yet alternatively, maybe it's something Lena finds on Axel's phone if she's on his route and pays him a visit in CH.13.
(although EK just trolling is perhaps the most plausible /s