have you looked at wuppy's tutorials there are some on custom dimensions that might help you.
http://wuppy29.blogspot.nl/2012/08/forge-modding-132.html
they are for 1.3.2 but they should still work fine for 1.4.2. also if you can look at some of the minecraft source to see how they have done it. with forge look in the gameregistry package and if i remember correctly there was something on biomes. anyways should be enough to get you started any problems let me know.
Make sure that all the cases are correct when i was writing mine i had the wrong cases on some of my files causing it not to work.
also remember that the files are referenced from the src folder not the current folder the java file is located.
for example urs should be located at %mcp%/src/Zeuscraft/ZeuscraftTerrain.png
hope this helps or atleast gets you on the right track. programming take persistence and patience as i have found wish you good luck in your mod
The easiest would be to create an xml file or yaml which alot of other mods use. If ur not sure how to write to an xml or yaml file you can just search for java tutorials tyere are quite a few out there. Hooe that helps u out, good luck