If you're referring to the white space above and below the image, there is nothing you can do about it. The table adapts to the size of the text. You can increase the width of the image, but it will decrease the size for the text, which means it will have to go down, and increase the white areas. You can change the color from white to what you want, if it makes it any better. Just edit the code of the color of the table's image cell.
Welcome, Frozen Cupcakes! I am much obliged for having you join us. To begin, I would request that you visit the Policies page to know what not to do in the wiki. There is also a Manual of Style, that serves as a guide for users wishing to create pages.
I recommend you browse through our various pages to grasp the general style of the website and to answer any outstanding questions you may have. If you require help with Japanese translations, please do consult me.
I do them myself with a complicated process that I've invented with Paint and PowerPoint. If you want me to make some for you, just provide me images with a good headshot of your characters and I'll do the rest.
Yes, I'm fairly decent at making backgrounds. I've been currently testing how different sizes of backgrounds affects what people see in different resolution screens, because in this wiki the background was mostly black for small screens (when the black was supposed to be placed behind the foreground).
By "stuff in the middle", I assume you mean the middle section of the image. Here's the image. http://hunter-x-hunter-fanon.wikia.com/wiki/File:FTFB.jpg. I'm happy with the result, but it's your opinion that matters. In bigger screens it shows Natsu/Lucy and in smaller ones Grey/Elsa.
Also, I don't add backgrounds through Theme Designer, it requires a very small file size and that requires quality loss. You probably know what it is; it's through MediaWiki. If you check the MediaWiki:Wikia.css in this wiki and copy the Wiki background section, you just have to change the image file link to the one of the image and it's done. Just refresh the cache and sooner or later, the image will be there.