If it was ripped out of the game, then the images width should be 2048x (as with all login screens extracted directly from the game), either the person cropped it, its a screenshot matching the original uploaders emulator/phone resolution, or its been uploaded by the devs on their official sites and reuploaded elsewhere where it should have a resolution of 1920x1080, however those tend to have watermarks on them. Also, ja this would be Suisai's drawing, he's done quite a lot of the old login screens.
Edit: And just uploaded the actual extracted image it to the wiki. Check any of the respective girls pages if you want it.
I updated the Source to the Twitter post from the official Girls' Frontline EN Twitter account. The picture there is also in 1920x1080, if someone wants to replace it.
Edit: oh, I missed that this one is without all the watermarks, unlike the one on Twitter.