Having problem with uploading images

I'm trying to upload an image that is going to present our adventure map, but even if it is 800x476 and 428 Ko, the image seems very small and I can't even modify his size. Seems like a very common issue, but I don't know what to do now...Only want it to be 4x time bigger at least since it's the first thing that will catch the player attention. Feeling a little stupid here...

[url=http://www.airshoes-warehouse.com/]Wholesale Air Force 1 Shoes[url/]
[url=http://www.shoptouk.com/] Louis Vuitton Clothing Online[url/]
[url=http://www.gochinashop.com/]wholesale gucci clothing[url/]
[url=http://www.coolclothingshop.com/]cheap gucci t-shirts[url/]
[url=http://www.oakleystyle.com/]Oakley Sungalsses Outlet[url/]
[url=http://www.safetoclothing.com/]replica louis vuitton t-shirts[url/]