I only downloaded the A3 poster.

The background bitmap needs two things. One, it is too low of resolution. It is 15 dpi.

Second, there is no bleed or not sufficient bleed. Typically you would want 3 mm of bleed that you can set up in Options | Page Size tab, about half way down. Then the elements that need to bleed (the background) needs to extend to the bleed lines you will see (solid red line around the page).

The shadow of the guitar will be trimmed off at its bottom if that matters (it wouldn't to me for the design).

As for the amount of bleed, you should check with whomever will be printing it.

The background gradient would be better off recreated as a vector element versus attempting to increase the low resolution.

Mike