You should contact an IP attorney to discuss the specifics of your claim and whether you can sue over the alleged copying. Generally speaking, one does not own a copyright in text, images, blending, color changes and typography. Rather, one owns a copyright in the creative expression of those. So to the extent you have taken non-copyrightable elements and combined them into a copyrightable express, you would have a case of infringement if someone else copies your work. If you cannot locate a local IP attorney, feel free to contact me.
Victor - email@example.com
NO, you cannot sue them and were very foolish to send a CDL yourself. You need a copyright lawyer immediately before you do something else dumb. Under 28 USC 1338(a) the US District Courts have exclusive jurisdiction to hear copyright cases. http://www.law.cornell.edu/uscode/text/28/1338 And under 17 USC 411(a) registration [or application for registration] is required. They know you don't have one so that is why they asked. You screwed up by sending that letter without first obtaining a copyright registration. Now you will be limited to actual damages which will be insufficient to justify the suit. Had you obtained registration, you might have a claim for up to $150,000. If you had consulted an attorney instead of going cheap you might have made thousands. Now you will get nothing. Sorry. Next time see a lawyer.
If you persist with your CDL now, unless you get permissions for the images, you may be on the receiving end of a copyright shakedown letter for the images you stole. Sounds like the other side is laughing at you, and with good reason.
L2BL: Not getting a lawyer can cost you thousands of dollars.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.