Article in English: Most images that you download online are already compressed. The file formats .jpg, .gif, and .png are all forms of compression, and further compression will lead to a sharp decrease in quality. Compression for images is most useful if you are working with images directly from a digital camera or .bmp files. It is not possible to uncompress a compressed image. You should always save the compressed version as a separate file so that you don't lose the original. There are essentially two types of compression for images: "lossless" and "lossy". Lossless compression is for cases where you need an exact replication of the original and is typically used for drawings, diagrams, and medical imaging. Lossy compression is for cases where a drop in quality won't be noticed as much and is most commonly used for photographs.  .gif, .tiff, and .png are all lossless formats. .jpg is the most common lossy format for images. Each image editor is a little different, but generally, the way you open a file is usually similar. Use the following steps to open an image in your image editor.  Click File in the menu bar. Click Open. Select an image. Click open. This will allow you to save a copy of the original image using a new format.Use the following steps to open the Export menu:  Click File. Click Save As (Photoshop, Paint) or Export As (GIMP). This saves your image in the new format. Image editors like Photoshop and GIMP will ask you to select your quality and compression settings after clicking the "Export/Save" button. You can typically use a slider to adjust the compression or quality. Increasing compression (decreasing quality) will result in a much smaller file size, but will add a lot of artifacts and change the color of the image. You may need to play with the options to find a good tradeoff between quality and size.

Summary in Chinese:
了解什么样的图片可以被压缩。 了解“无损压缩”和“有损压缩”之间的区别。 在图片编辑器中打开图片。 点击“文件” → “导出为”。 从“类型”菜单中选择合适的格式。 设置质量选项（如果可以的话）。