Every content management system will accept a large image and show it. That is the problem: nothing complains, the page simply becomes slower, and nobody notices until someone measures it.
Why the order matters
A 4 MB photograph displayed at 800 pixels wide is not made faster by a plugin that re-compresses it on the server - the file still had to be uploaded, and the stored copy is still large. Doing it before the upload means the heavy file never exists on the site at all.
There is also a practical limit: many hosts cap upload size, and a phone photograph can exceed it. Doing the work first avoids that failure entirely.
The routine
- Resize to the display width. Card images 400-600 px, article images 800-1000 px, full-width banners up to about 1920 px. What image size do you need has the details.
- Choose the format for the destination. WebP for a web page, JPEG when compatibility matters, PNG only for screenshots and diagrams.
- Compress at a moderate setting. Check one image at 100% before doing the batch.
- Upload the results, and delete the originals from the upload folder so the right file cannot be picked by mistake.
It takes a minute per batch, and it happens once. The alternative - discovering the problem later - means re-exporting, re-uploading and updating every reference to the file.
Keeping the originals
Keep the uncompressed originals somewhere else: an archive folder, a backup, a camera card. Compressed files are the delivery copy, not the master. If the design changes and a larger version is needed, the original is the only way to get it back.
What this does not fix
Compression is not a substitute for serving the right size to the right screen. A site that sends a 1920-pixel image to a phone is still sending more than the phone will display. Fixing that needs responsive images in the markup, which is a separate job from preparing the files.
Doing it now
The compressor handles a whole batch at once, keeps each file's name and shows the real before and after size for every one. Nothing is uploaded, so the images never leave your device.