Creative help needed please :)

So at work I need to find some images for a board thats just a bit bigger than A1

I need two main images.

One is a rainbow that will run from one corner to another and one is a pot of gold.

I've had a look and found some good images but with an A4 printer not sure how I would get the images to be split so the board won't look rubbish any suggestions?

Also if anyone knows where I can find some fancy pictures that would be great as i'm looking to impress

Print small rainbows, postcard size.and build your own.