First thing is upload your new image here simplicity/templates/gk_simplicity/images/yourimage.jpg
Then you will need to edit this file simplicity/templates/gk_simplicity/css/system/error.style1.css Or whatever style you use, download the correct one.
Open this in your editor, Near the top you are looking for this
body {
font-family: 'Open Sans', Arial, sans-serif;
min-height: 100%;
background: #fff;
font-size: 16px;
line-height: 2;
color: #777;
padding: 0;
font-weight: 300;
word-wrap: break-word;
}
You are going to replace background: #fff; with background: url("../../images/yourimage.jpg") no-repeat scroll center 0 / cover #fff;
So he is what it will look like
body {
font-family: 'Open Sans', Arial, sans-serif;
min-height: 100%;
background: url("../../images/yourimage.jpg") no-repeat scroll center 0 / cover #fff;
font-size: 16px;
line-height: 2;
color: #777;
padding: 0;
font-weight: 300;
word-wrap: break-word;
}
So the position of the image could be off. I dont know the size of your image so cant tell where it will be. If you pm me or post your url after you add the code I can tell you how to position the image where you want it.