Board Thread:Support Requests - Designing Your Wikia/@comment-5790751-20140605232800

As the title says, how would I change the background picture of my wiki to be a picture larger than 300kb? When I try to upload my desired picture, it says the size is incorrect. Any alternative pictures that are under 300kb are to small to fill my entire background, but I see other wikis that have full page pictures as their background. Is it because they pay extra money or something? 