お手持ちの画像がディスク容量を圧迫しそうなとき、 画像の寸法を変えずに分量を減省できる方法があります。 最大級の画像圧縮は JPEG 形式を用いれば可能であり、 画像が既にこの形式で保存されていても JPEG 形式のもつ順応性の高い圧縮指針のおかげであとから圧縮率を変えて保存し直せるので大抵はさらに分量を減らせる余地があります。 ただし画像の容量を減らせば減らすほど元画像の精確さが失われるという交換条件があります。 しかも JPEG 形式の画像は保存し直すたびに画像の質が落ちてゆく性質があることも知っておくべきです。

Images are loaded and saved as .XCF files. Your JPG image has been loaded as XCF. GIMP offers you to Overwrite image-name.jpg or File → Export As to open the 「Export Image」 dialog.

The dialog opens with the file name already typed in the Name box, with the default .png extension. Delete the existing extension and type JPG instead, and GIMP will determine the file type from the file extension. Use the file extension list, marked with red in the figure above, to see the types supported by GIMP. The supported extensions change depending on your installed libraries. If GIMP complains, or if 「JPEG」 is grayed out in the Extensions menu, cancel out of everything and step through the 「モードを変える」. Once you have done this, click Save. This opens the 「Export Image as JPEG」 dialog that contains the quality control.

The 「Export Image as JPEG」 dialog uses default values that reduce size in memory while retaining good visual quality; this is the safest and quickest thing to do.

Reduce the image Quality to make the image even smaller. Reduced quality degrades the image, so be certain to check 「Show preview in image window」 to visually gauge the degradation. A Quality setting of 10 produces a very poor quality image that uses very little disk space. The figure below shows a more reasonable image. A quality of 75 produces a reasonable image using much less disk space, which will, in turn, load much faster on a web page. Although the image is somewhat degraded, it is acceptable for the intended purpose.

最後に同じ画像を使って圧縮率をいろいろに変化させた比較をご覧ください。