Open your layout in Photoshop.
File >Save for Web.
Make sure the Optimized file format box says JPEG. Change if necessary through the dropdown box.
Set Compression quality box to Very High. This will give you the best image quality when you shrink down your file.
Under Image Size, change Width to 800 px. Note: The Length size will change automatically to keep the proportions correct. Hit the Tab key. You’ll see the image in the preview box change.
You can see the size of the document in the bottom left corner of the preview box. If it’s too large for the size limits on a gallery or blog, you can make the image size smaller or reduce the compression quality slightly.
Click Save. Navigate to the folder where you want to keep it and give the file a name that will help you remember it is the web-sized version. For example, type “MyFilename-web.”