How to Compress PNG and JPEG Images with an Online Tool

An online PNG and JPEG compressor makes image files smaller so they upload faster, fit attachment limits and load quicker on web pages. This guide covers preparing your files, choosing settings, checking the compressed output and fixing the problems people run into most often.

Before you start

Keep an untouched copy of every original. Compression is usually one-way: once detail is discarded from a JPEG or colors are reduced in a PNG, you cannot recover them from the smaller file.

• Know which file format you have. JPEG suits photos and camera snapshots. PNG suits screenshots, diagrams, logos and anything that needs transparency.
• Note the pixel dimensions of the original. If the image is much larger than the size it will be displayed at, resizing often saves more space than compression settings alone.
• Check any target limits you need to meet, such as a maximum file size for an upload form or a required width for a website.
• Remove or avoid uploading images containing sensitive content, such as documents, faces or screens with personal data, until you have confirmed how the tool handles files.

Find out whether the tool processes images locally in your browser or uploads them to a server. Do not assume either. Read the tool's own description and privacy policy for statements about uploading, storage and deletion of files. If the policy is unclear, treat the images as uploaded to a third party. Also check whether the tool keeps or removes metadata such as camera details, date and location, since some compressors strip it and others preserve it.

Step-by-step instructions

• 1. Open the compressor in an up-to-date browser.
• 2. Add your PNG or JPEG files using the file picker or drag and drop. If you have many images, check whether the tool supports batch processing and any limit on the number or size of files.
• 3. Choose the output format. Keep the original format unless you have a reason to change it. Converting a PNG photo to JPEG can shrink it considerably, but JPEG does not support transparency.
• 4. Set the compression level if the tool offers one. For JPEG this is usually a quality setting: lower values give smaller files with more visible artifacts. For PNG, tools may offer lossless optimization, which keeps every pixel, or lossy color reduction, which reduces the number of colors for a much smaller file.
• 5. Resize if needed. Enter the target width or height and keep the aspect ratio locked so the image is not stretched.
• 6. Start the compression and wait for the tool to show the result.
• 7. Download the compressed files and save them under new names or in a separate folder so they do not overwrite the originals.

Start with a moderate setting rather than the strongest one. If the result looks good, you can compress the original again at a lower quality. Always recompress from the original rather than from an already compressed copy, because each lossy pass adds more artifacts.

Check the output

• Compare file sizes of the original and compressed versions in your file manager or the tool's summary.
• Open both versions side by side at 100% zoom. Look at fine text, sharp edges, skin tones, skies and smooth gradients, where compression problems show first.
• Confirm the pixel dimensions match what you intended and that the image is not distorted.
• For PNG files, check that transparent areas are still transparent by placing the image on a colored background or in the program where you will use it.
• If metadata matters to you, inspect the file properties to see whether date, camera or location details were kept or removed.

Keep the format limits in mind. JPEG compression is always lossy and tends to blur hard edges and text. Lossless PNG optimization cannot shrink a file as much as lossy methods, and a large photo saved as PNG may stay large regardless of settings.

Common problems and next steps

The file barely got smaller. The image may already be compressed, or a PNG may contain a photo that the format handles poorly. Try resizing to the display size, using a lossy PNG mode, or converting a photo without transparency to JPEG.

The transparent background turned white or black. This happens when a PNG is converted to JPEG, which has no transparency. Keep the output as PNG if you need a transparent background.

Blocky artifacts, halos or banding appear. The compression is too strong. Go back to the original and use a higher JPEG quality or a less aggressive PNG color reduction. Screenshots with text usually look better as PNG than as JPEG.

The upload fails or the tool rejects the file. Check that the file is a genuine PNG or JPEG rather than another format with a renamed extension, and that it stays within the tool's stated size or count limits. Very large camera images may need resizing first.

Colors look different after compression. Some tools discard embedded color profiles along with other metadata. Compare the result in the application where the image will actually be used, and try another tool or setting if the shift matters.

If online processing is not acceptable for your images, use a desktop image editor or an offline compression utility instead, applying the same principles: keep the original, choose the right format, resize first and compare the result before replacing anything.