Auto Resize Images - Regardless of HOW they are Added

Morning all...
I'm trying to find a plugin, function, something to run as a cron (anything really) that will resize recently added images to the maximum size set in Wordpress settings.

Easy enough when someone adds a photo via the normal Post/Page routine but when using a completely custom theme / plugin that does the uploading, the photos bypass the WP size check and remain at the original size.

I was thinking of telling all users of a new site to make sure images are no bigger than "---" etc but that's in the poor usability list of no nos!

An example is using something like PremiumPress classifieds where the user adding an item just uploads their photos without any checking of size. I've tried a few resize plugins but nothing, so far, is resizing photos uploaded via the front end.

I suppose it's like uploading a photo to Facebook where it doesn't matter what size gets uploaded via the front end, it gets resized.

One method that crossed my mind was having a bit of code in the theme that checks the size of an image file when it is requested and if it exceeds maximum size, resize it, save it and carry on.

Anyone worked on something like this in the past?