...the buttons only have one problem in reference to buildings and units
you can custom everything except the "created/built" pic event (round image with unit/building popup - then disappears - when the turn starts and the unit/building is build) and the offscreen for custom units....
apparently these images are contained in the separate atlas.dds files and can not be modded! (maybe a hardcoded issue - hasnt been solved so you get a "pink circle" when a unique building/unit is built... if the unit doesnt use vanilla Civ images)
the debate and discussion is also posted there...
the links also provide how to create buttons in the city screen ("option to build" button) the unit stack (small icons that lists the units in the stack) and city training/building bar in the city bar info:
http://forums.civfanatics.com/showthread.php?t=156760
http://forums.civfanatics.com/showthread.php?t=151450
adding civic buttons are a little harder because it modifies the python files as well as the (easier) XML files
go here for the complete tutorial on adding civics... it also details on how to add the buttons to the civic screen... and editing python files:
http://forums.civfanatics.com/showthread.php?t=138779
buildings and wonders are similar to units...
you just need to CIV4ArtDefines_building.XML files
just remember to create/edit a folder path that matches the path you put in the CIV4ArtDefines file... and place the .dds file in the appropriate folder...
*NOTE* REMEMBER TO BACKUP ALL FILES AND EDIT THE COPIES ONLY!
(Preferably making a mod (folder) for all your edited copies)
hope this helps...