you insert an
env_sprite into your maps entity list (using RipEnt - which extracts the entity properties/co-ordinates/keys/etc into a dos file, which can be editied through notepad
)
you key in the co-ordinates for where you want the sprite to appear in your map.
you set the sprites path to
sprites/yoursprite.spr. put the sprite in your server's sprite folder (using any ftp accessing program).
the client will automatically download the sprite and hey presto! in-game custom sprites.
the same goes for models, but it's easier to use a model that won't need clipping, which only complicates the matter further.