Image resize stand alone php for charity site

Hi. I run charity events and have applications for vendors but the images they upload are too big and its stopping the form submit. I want just a simple 1 php page where i can add a link on the form to it, then then click upload, select photos, click resize, then they list below converted and they can download them. My site is wordpress if you might know a plugin to do this instead. I dont want the single page integrated in the site just a single page php.