Second, marking your image can discourage inappropriate reuse of your image on the web. The size of your large image for web presentation should generally not exceed 350 pixels in the longest dimension. They sacrifice image quality for smaller image sizes. JPEG files are better for full color images and GIF files are better for limited, flat color. In most cases you will want to use JPEG format for your large images and GIF format for your thumbnail images.
Compression causes the file to throw out some image data.
Image quality and file size are reduced as the number gets smaller. This should be your starting point for picking the right compression. Do not repeatedly open and edit a JPEG file, each time you save it the image is degraded. Plan to do this as the last thing in you image editing for web use. This format works best with images that have a limited color range.