First let's look at a simple page like this.
What is wrong with a page like this?
The image is not transparent. Images that are transparent tend to make the page look better.
To make an image transparent you must either use an image editing program (ex. Photoshop or GIMP) or an online image editor. Make sure the file is not saved as a ".jpg" as that file type does not support transparency.
If you cannot make an image transparent, it is recommended to add the Category:Transparency Needed to the image's page so that other people who can make the image transparent can do so, and to make the file type ".png" so that it is easier to update.