You can make a smaller book cover image to use as your blog background

You have found the right artist to design your book cover. You debate the characteristics of the cover. Sketch follows sketch until you get the perfect cover. They send a file with the cover. You think it would be a great asset in marketing the book if you were to put the cover as your blog background. But your blog provider tells you that your file is, ‘too big’.
1. Use the Resize function in Windows Paint to create a smaller size version of the book cover image for the blog background.
2. You’ll want the maximum resolution that Blogger allows for quality purposes.

Here is some relevant info from the Blogger Team: “If you want a full-screen background, we recommend using an image that's 1800 pixels wide and 1600 pixels high, so that the background image can fill the entire screen even for the readers with large monitors. To make sure your blog loads quickly, background images must be small in file size. As the maximum file size you can upload is 300KB, you may need to reduce your photo’s file size using Photoshop, Mac OS X Preview, or one of many free online tools. You can also tile a smaller image, but keep in mind that patterns work better than photographs, which can make your blog look cluttered.”
The point about photographs applies to more generic blogs that use a lot of photos in their posts and thus there is confusion between the posts and the background. This may not apply to literary blogs that have more text than visuals in the body of the blog.

Earlier Sketch of Book Cover designed and executed by Chiara Castagna

Thank you to technology-expert Alex B for your advice in making a cover smaller so that it can be used as a blog background.