You can't edit the main games with the editor, no.
However you can easily replace any images in game. See:
viewtopic.php?f=3&t=4471That's on how to replace character portraits but it works for any image. The "data.pak" file in the game's install folder is a normal zip file. You can examine it to find out what images are there and where they are in internal folder hierarchy.
A small snag is that stuff in the world, like guards are transparent and have mipmaps. It might or might not work if you simply replace them with a bitmap (bmp) like with the heads. If the original is transparent, the replacement must be transparent too. So it must be a 32 bit BMP with alpha channel. If it crashes with that, the replacement must be dds with mipmaps.
There are programs to create such. One is
https://www.imgtec.com/developers/power ... vrtextool/To encode the image, select the "DirectX 11" group (any) and "BC3 Linear RGB" format and check "Generate MIPMaps".