When producing new versions of any graphic, always use the original to make each new version. This will retain as much of the quality as possible. Always ensure that the image mode is on RGB color.
Each time a graphics package shrinks an image, it reduces the number of colors, therefore reducing the quality. So if you use the original every time, you will always get the most colors possible.
Unless the image is a vector image, you can't create a good quality larger version as there won't be enough colors to render the image at the same quality.
Once a graphics package, such as Adobe Photoshop or Adobe Fireworks, has optimized the graphic, you cannot get back to the original quality. GIFs are indexed color and JPGs may have their quality reduced to shrink the file size.
There will always be a sacrifice between quality and file size. The smaller file size will result in faster download times.
Never use the width and height attributes of the < img > tag in the HTML of a web page to resize images. If you use this method to reduce the image size, you will increase the download time of the page as a whole. This is because the browser has to "re-draw" the image at an incorrect size which takes longer.
Avoid using the same technique to increase the size of an image. It severely reduces image quality as well as taking longer to download. Use your favorite graphics software to re-size your images properly and include the correct width and height dimensions for a faster download. The result will be a better-looking website as well as faster download times. Kraken.io and Optimizilla are a few of the free online tools you can use to compress and resize images. Take note: the tools mentioned are not affiliated with Bluehost, you can still research online for other tools that work best for you.