Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 132

Figure 2.58
The Save as JPEG
dialog
When you click Save , you will notice a dialog asking you to export the
image as you save it. This ensures that the original will not be overwritten.
Click the Export button.
Saving to JPEG—the Actual JPEG Compression Dialog
At this point, the dialog for setting the file compression will appear. Click
the Show Preview in Image Window check box so you'll be able to see the
changes to the quality in the image window as you use the slider to adjust the
compression. Zoom into the image to better see the compression artifacts
(squares in which the colors are heavily unified) occur.
Settings for Advanced Options
When you click the Advanced Options button, the Save as JPEG dialog expands
to show additional options. Here are some guidelines to using these options:
• Make sure the Optimize option is checked. This improves the ratio
between the quality/compression and the file size (so that as the file
becomes smaller, the quality remains the same).
• Select a compression method in the Subsampling drop-down menu.
1 × 1 is a good standard for small file sizes.
• Selecting Floating-Point from the DCT Method drop-down menu provides
the most exact method for calculating the compression. Although it is
the slowest, it results in the highest quality.