I saw more than a few people wondering about this stuff, myself included, and there's not really a comprehensive guide that I can find so I figured I'd write one up. The main problem I had making my texture pack was figuring out where all the block textures were actually being gotten from. In your IC2/sprites folder there's several files. Lets start with the block_0.png. Not every texture on here is actually used, some are just on there because they're left over from previous versions. as of the latest version, all the unused stuff has been taken out.
Starting at the top row, from left to right, you have the iron fence texture, reinforced stone, reinforced glass, reinforced door top and bottom.
On the second row is the old texture for machine blocks bottom and side, which is used somewhere that I can't figure out, Next is the top of the booze barrel, the tapped tree-hole (for the barrel side), and the luminator at the very end.
On the third row there's copper ore, tin ore, uranium ore, mining pipe sides, mining pipe top/bottom, construction foam, rubber sapling, rubber sheet, sticky resin (when placed), rubber tree side, side with sap hole, side with empty hole, and top/bottom.
Fourth is the remote activated dynamite and dynamite, the industrial TNT, and the nuke.
The fifth row is the reactor chamber, the copper block, and the top/bottom of the uranium block.
Sixth row is the bronze and tin blocks, and the uranium block side.
The seventh row is all of the painted construction foam, with the unpainted/white one last
On the eight row is the scaffold sides, top, and reinforced scaffold. Directly under those are the iron equivalents.
Block_cable is the most confusing one, for me anyway. This contains all the textures for the cabling, but you don't need most of the stuff on here. Here's the stuff you need, taken from my texture pack files (If you need it at 16x just shrink it by half). The textures are actually arranged in a + shape, and depending on which wire it is, may or may not use the whole block.
From the top row down it's insulated copper, uninsulated copper, gold, insulated gold, 2x insulated gold, HV cable, 1, 2, 4x insulated HV cable, and fiber cable. The different colored ones in the rows are for when you paint them. The two under those are the redstone cable and splitter cable. The construction foam textures in the default are copied onto here in the row after that, I assume for foam sprayed cables. In the default file the vanilla colored wool textures are at the very bottom, but they seem to serve no purpose that I can determine.
The rest of them are fairly easy, and all set up the same way. Here's an example from the block_machine:
The top half is when the block is inactive, and it switches to the bottom half of the texture when it's running or active. This is by far my favorite part of texturing for Industrialcraft, it lets you have a unique texture for every side of the block, not just one or two, and change all of them when the block is active.
The columns in block_machine are labeled in the image above.
Block_electric is all the power transformers and storage units. From left to right it's the batbox, MFE, MFSU, LV transformer, MV transformer, and HV transformer
Block_generator is the generators: Generator, geothermal, water, solar, wind and nuclear. Keep in mind this is just for the main reactor, the reactor chambers are still in the block_0.png as noted above.
block_machine2 is the teleporter, tesla coil, and crop matron
Block_personal is the safe and trade-o-mat.
If you have charging benches installed they're done the same way as well.
The only other images are the GUI images, which are pretty self exlpanatory, the item_0.png, which is your items, and the laser.png which is, well, the laser beam texture.
The only other one is the crops, and I can't even figure out how to grow them, much less what the textures are, so you're on your own there.
Let me know if this was helpful, or if there's any questions.
