To crop your image so that your cake is right in the center and as large as can be, you can use your own photo editor (such as Photoshop) or this easy-to-use online photo editor.
Here are examples of images before and after being cropped:

Before being cropped...

After being cropped...

Before being cropped...

After being cropped...

Before being cropped...

After being cropped...

Before being cropped...

After being cropped...

Before being cropped...

After being cropped...

Before being cropped...

After being cropped...

Before being cropped...

After being cropped...

Before being cropped...

After being cropped...