1. Work out what the destination needs
Check the upload limit, accepted formats, and requested image dimensions. These are separate requirements: a file can have the correct width but still exceed a size limit. Write down the target so you can stop when the image meets it.
Keep a copy of the original in a separate folder. Give trial exports clear names, such as landscape-small.jpg and landscape-medium.jpg. This makes comparisons easier and avoids accidentally replacing the source you might need for a larger version later.
2. Resize before fine-tuning quality
If the destination asks for a 1200-pixel-wide image, begin with an export at that width. Keep the original proportions unless you intend to crop. Resizing and cropping solve different problems: resizing changes the dimensions, while cropping removes part of the composition.
Preview the resized image in its intended context. Read any text and inspect the subject. Avoid enlarging a small source merely to reach an arbitrary number; judge whether the available image has enough detail for the intended placement.
3. Compare formats and adjust gradually
For a photo, try JPEG or WebP. For small text or a diagram, include a PNG version in the comparison. Use the same source and dimensions for each trial so you can assess the effect of changing the export settings.
Browser image export can use a quality value for lossy formats such as JPEG and WebP. That value is an encoder setting, not a promised percentage reduction in file size. PNG export does not use this lossy quality control. Browser support also affects which formats can be exported.
Further reading: MDN: browser image export and quality settings
4. Inspect the file you will actually share
Open the downloaded result and compare it at the same size as the original. Look for blurred lettering, blocky shadows, uneven gradients, and halos around edges. Also check transparency or the chosen background color. If an important detail suffers, raise the quality or reconsider the dimensions.
Check the actual byte size against the upload limit. An already compact image may not get smaller after another export. If a new version is larger and offers no useful improvement, keep the existing file. Finish with the smallest version that satisfies your destination and your visual check.