How do I optimize images I already uploaded?

2 min read · Last updated June 2026

Go to Media > Bulk Optimize and click Start Bulk Optimization. SlashImage works through every image already in your media library in the background, so you can leave the page while it runs.

Prerequisites

Your API key is connected and the plugin is active.
New uploads are already optimized for you, so bulk is only for the images that were there before you installed SlashImage. See How do I automatically optimize new uploads?.

Run a bulk optimization

Go to Media > Bulk Optimize.
Click Start Bulk Optimization. The page switches to a running view with a progress bar and counts for optimized, skipped, and remaining.
Leave it to run. SlashImage adds your images to a queue and works through them a few at a time in the background.
Use Pause and Resume to control the run, or Cancel to stop it. Images already optimized stay optimized.

The run is done when the remaining count reaches zero. SlashImage skips images it has already optimized, so you can start another run any time and it only touches what is left.

Re-optimize images you already did

By default, bulk skips anything already optimized. To redo them with new settings, tick Re-optimize the [N] already-optimized images before you start. Its help text reads "Reprocesses them with your current compression settings. Uses additional credits."

What can go wrong

The run pauses when you close the tab. If your host has WP-Cron disabled, the page shows: "WP-Cron is disabled on this host. Keep this tab open while bulk processing runs. If you close the tab, processing will pause and you'll need to return here to resume." Keep the tab open, or reopen the page to resume.
Optimized images still load at full size. Clear your page cache and CDN after a big run so visitors get the new WebP and AVIF versions. See How do I use SlashImage with a CDN?.
Some images show an error. Look the message up in the image error reference.