Many thanks @rvietor, that’s been really helpful - I’ve learned a bit about layer masks and I’ve been able to create a transparency gradient between two images, as below. Layer 1 is my original cabinet image. Layer 2 is the colourful triangles image - with a layer mask added, and a black to white gradient applied across it.
However, I don’t know how to create gradient which is not based on a second image. i.e. it’s not an image in the second layer, but just a brightness gradient mask - how do I do this?
Thanks again!