A lot of the VLD fandom have pointed out the placement of the current paladins and how they are positioned in front of the previous paladins: such as Keith standing in front of and in between Shiro and King Alfor (symbolising that Keith has been the pilot of both the black and red lions - same for Lance with the red & blue lions).

SO, from this, we know that character placement in this image is important.

Now to Lance and Allura. Lance is in between Keith & Allura. When reading the image from left to right, Lance is ahead of Allura. Lance's face is similar to when Keith left to fight with the Blade of Marmora (we have seen this previously in the show - when Lance smiles sadly he has slight lines under his eyes - possibly to indicate stress or worry). 

To me this is the face of someone having to say goodbye, be strong and move on. Lance is facing forward, but looking back, perhaps symbolising a regret. You can also see there is hesitation or slowness in Lance's movement, as compared to the other current paladins, who walk with strong strides and purpose (shown by their arms and legs), Lance's legs are closer together and his arms close to his sides. Lance is sadly moving on from Allura, but moving on none the less.

Allura is placed behind Lance (from left to right). She, like the other current paladins is in her new uniform in front of the previous blue paladin, but unlike the others she is holding her helmet. Being at the end of the group and at the far right side of the image puts her almost apart from the others. The fact that she is wearing her new uniform and holding her helmet, while looking in a different direction to the others (apart from Lance who has noticed her gaze) I think symbolises how Allura herself will also be moving on.

She has always been Princess Allura; one day destined to rule & be Queen - her path will lead her to be ruler of a new Altea.

I also think that, as Allura is looking back, there has to be another final piece to this poster, perhaps released after the series has actually aired. If this is to be the final, final season, the final image will show what happens to the team at the very end. At least, I hope so.