First, this is a trademark issue, not a copyright issue. I believe someone already posted what I consider to be the most accurate answer to the OP, which was something along the lines of "if you're using their trademarked logo as a means to make a statement about your product, then there will be legal issues." But, since you're not a well-known artist (and I assume you're not signed to a major label or any label), IF Marshall noticed the use of their trademark, and IF they felt it was inappropriate enough to take action, the worst thing they can do is issue you a cease-and-desist letter, threatening further legal action if you continue to use their logo (they'll probably say that you're "misrepresenting" their company with your particular use of their logo). But, the only other legal action they could take would be to sue you for damages, which they're not gonna do because you most likely don't have enough assets to make such a lawsuit worthwhile.
Now, when I was signed to Virgin Records a few years back, ANY & EVERY photoshoot we did, whether it was for promotional purposes, an album cover, a magazine/print material, etc., was supervised by a rep from Virgin who was there to ensure we didn't include ANY logos of ANY sort in any of the photos. T-shirt with a random design on it? Cover it up. Jean jacket with a Ramones patch on it? Cover it up. Any sort of corporate logo in the background of the shot? Photoshop it out. They did this because they're a HUGE, wealthy corporation with plenty of assets to go after if any of the owners of the particular trademarks we wore decided they wanted to sue.
I'll do a few different ones and chose which one I like best. I not bothered about it being the best album cover of all time but do want it represent at least the right type of music.
