So firstly with Rat its the most (I don't have favorites shhh) He's got Lux's goggles as well as a shift in style to more needs based with a bag, hair pulled up, gloves, and a bandage + eye bags thrown on to hint at some of the heightened struggle since Lux was gone. His pose is referencing the inspiration with the hand dragging but that is also specifically Rat not letting go of Lux while trying to move forward (into the rising sunlight) His face is specificlly visible because he's attempting to move forward (even if only barely) and is wanting to continue on in honor of Lux/for Lux. Same meaning with the light being a morning a new day, potential and all that.

For Fundy its more simple, this is new information, he's only just shown up, this is the first image he's seen of his sister since he lost her and he is distraught and ofc who can blame him. he's crying reaching out for her despite it only being an image. Fundy is covering his face, hiding from the reality intentionally or not, although its not set in stone he's facing closer towards the setting sun, towards the night, and away from progress in his distress. With his head in his hands and the low light you couldn't be able to tell for sure if the sun is rising or setting without the context of Rat and Winsweep's pieces.

And finally Winsweep, he is facing definitively away from the "camera" he's frozen in a stiff pose rejecting mourning where the others are accepting it (more or less). He's also stiff in style, acting as tho he is unchanged by the loss he remains in his nearly comical vest and clothes. He's hiding himself from the shame and he's in the dark in several ways. Firstly its night time and secondly even the light from the moon doesn't hit his face, additionally he's hiding Lux's face from view as he portrays the version of her he wants to remember and wants back. A version not seen in the portrait/painting/photo of Lux.

Lastly Lux's portrait, I think she was truly happiest with Rat and Astron at the cabin. She was welcomed not as a tool for the first time by them and so that's the portrait that remains of her. The small bit of true unhindered joy she had between her hair going white and her losing her arm. So of course Rat and Astron would want to immortalize the image of her when the remember her best, when she was happy and as free as anyone on the server could be.