Firstly, in the future, keep in mind we have a forum section for questions like this. User2User Tech.
Is it giving you an error telling you the picture is too big? Is it a gif, jpeg or bmp? Are you uploading it from your laptop, phone, tablet? Or are you linking it from another site?
If it's not giving you a message and you simply read the requirements without trying, is suggest trying it anyways. Usuly it will resize the picture.
If uploading and it is giving the error message then I believe there are websites that allow you to make those sorts of changes for free. I don't know of any, since I use Photoshop. Perhaps another user will be able to provide a name. Or you can Google search for them.