Before things start getting silly, please take note of the following.
As a designer myself, it is fair to say that no design is truly original. We all take our cues and inspiration from various sources and then we use our own creative flair to incorporate elements and create our own designs. Taking inspiration from others designs and making them our own is how design works, however, as with plagiarism, the rule of thumb is that you change enough of the design you are ripping off to stop it being easily recognised as a blatant copy.
If you are using someone elses design and tweaking only a few aspects of it e.g. background & fonts; then state that you are doing so or credit the original as the source of your inspiration! However, this is not a requirement of the competition, I am merely stating common sense/decency.
I'm not going to get into copyright disputes as this is meant to be a light hearted and fun competition and it makes zero difference if the resulting art work is better or worse than the original being copied.
We are not stopping you ripping off each others work or taking inspiration from other peoples work, but where it is blatantly obvious that this has been done, this will be taken into account when we judge the entries.
Now, can we all just play nice please?!!