Images can be downloaded directly from the game if they are accessed while the browser's Web Developer Tools (F12 or Control+Shift+I) is open for the game's webpage. Click on the image files that you want from the list and they'll open in a new web page, right click on the image, and select the 'Save Image As' option. In the case of soul skin portraits, there's a 960x640 image, the one included in the GIF archive and the one in the screenshot, as well as a 1440x960 image, which I don't know of a way to get without buying the Premium Pass or waiting for the Japanese wiki to update. As far as I can tell, there's also no section on the Japanese wiki for Memorial Illustrations, so the only way to get the thumbnails is to download them directly from the game.