The RPC jet is to boring for me and i wanna change it but every time i replace the jet thing with the image i want to come out it says. Image (name of image) is undefined. What am i doing wrong?

What are you referring to in the RPC jet? Is it an animated sprite and thus needs more than one frame? Are you pathing to it correctly?

uhhhhh. what?

What is this RPC jet you refer to?
If you're talking about the Rocket Propelled Chainsaw, then the jet is a MuzzleFlash, which is an animated sprite, IE, a collection of images rather than one single image. So, you must either make the exact same number of frames as the original or change the number of frames.
EDIT: Also, you have to path to the image correctly:
If you have the image(s) you want to path to in (mod).rte/images/sprites/, you have to tell Cortex that they are at (mod).rte/images/sprites/(image).bmp

Yes i mean the RPC. I'm not 100% sure how to make it change. How would i tell cortex command to change it to these images