Unable to upload textures(solved)

So in the web interface, I uploaded a few images, to use as bitmap textures(a few banners, nothing big or special).

However, when I'm editing the world, I'm not able to place said "asset" anywhere. The "place asset" menu item is always grayed out. Thumbnails never render, showing the "blue cube" icon. 

Any way to fix this? I would like to apply a few pictures to my world to give it a bit more realism.