I agree that without any idea how it will be used, it's silly to make it a requirement. I was going to ignore it until I saw examples and such, but then they changed things so you *must* include one when you try to change your app.
What's needed is a description of the context in which it will be used, whether it will ever be scaled down and, if so, by how much, and perhaps most usefully, some screenshots showing an example in the appropriate context.
Without that, we risk making images with features that are too small to survive scaling-down to whatever size they actually use, and we risk things like having it clash severely with the background or surroundings, possibly defeating the whole purpose of it showing off our apps.
I know one dev whose app is portrait-only mode, and he found it effective to shove in three screenshots, side-by-side. I fear it may not scale down well, but we did check it at one-quarter size and it would be okay, if not optimal.
For mine, I simply scaled up my square 480x480 image (from the vector source of course) and threw in the app title on the left, rotated sideways, and a bit of the text from my 50-char description on the right. Gave it some black/white border combo so it at least stands out from whatever background it will be on. I bet it looks like absolute **bleep**, but I wasn't going to invest a lot of time with no idea what I should be doing.