First you'll need to open a browser and visit your screenshots.

steamproxy-script.pipiskins.com/my/screenshots

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Select the game that you want to delete the screenshots.



Click manage screenshots then scroll down until you can no longer scroll down.



Right click anywhere on the page, hit inspect and go to the console. ( or ctrl + shift + i )



Copy / paste this, hit enter and everything will be checked.

$$('input[type="checkbox"').map(i => i.checked = true)

https://pastebin.com/DtWvfBjw

Hit delete then this error will appears, uncheck and check one of the screenshots, it will be fixed.



It will take a while to delete everything so be patient.

Oops, i forgot to mention that zooming out of the page to 25% will help load all the screenshots faster.