A practical order of operations
Make a copy of the original. Resize it to the largest size the page or document needs, taking responsive layouts and high-density screens into account. Smaller dimensions mean fewer pixels to encode and often yield a substantial reduction. GZ's free JPG compressor lets you adjust dimensions and quality, then inspect the generated file size before downloading.
Then compare quality settings using representative areas: faces, hair, text, foliage, shadows and gradients. Don't rely on the setting number alone; different encoders use different quality scales.
Choose JPEG or another format
Keep JPEG for a photo when compatibility is important. WebP can be a smaller delivery choice for supported browsers, while PNG may be useful when avoiding another lossy pass matters more than file size.
A JPG and a JPEG are the same image format with different filename extensions. Compression does not mean the original is overwritten in GZ; the tool downloads a new copy.
Frequently asked questions
Can GZ reduce a JPG to exactly 100 KB?
There is no exact-size target. To get a JPG under a 100 KB or 200 KB upload limit, reduce dimensions first, then lower quality a little at a time and check the actual output. A detailed photo may not look acceptable at a very small target.