If you have an image that you want to resize in Illustrator, there are a few things to keep in mind.
First, make sure that your image is in the correct format. Most images that you must use in Illustrator should be in .ai or .eps format. If your image is not in this format, you will need to convert it before you can resize it. Or you can manually resize the image by adjusting its default width and height. However, the image will likely lose some of its quality unless it is in the appropriate format mentioned above.
Second, make sure that your image is sized correctly, to begin with. Most images that you must resize in Illustrator should be in .ai or .eps format. This is because other image formats tend to have a very large or small file size which drastically affects graphic and web performance. When you scale an improperly sized image, you might end up having a distorted resized image such as shown below.
Finally, be aware of the resolution of your image. Images that are exported and imported into Illustrator usually appear blurry and pixelated. This is a common issue and you can avoid this by understanding the most essential part of an image, which is the resolution.