If you have the ImageMagick module installed on your server, I can send you an 'image optimizer' I wrote in PHP.
It won't 'completely' create your banners, but it will give you a good starting point as to how you can resize images via ImageMagick and php... just change the percentages to your exact banner sizes ... 468x60, etc.
All you'd really need is a couple 'generic' banners in a directory and you could also use ImageMagick to let the user add text to the actual image.
if you want to create banners on a budget rather then spend the big bucks on a program like photoshop you can go get the gimp. Great software, not the easiest to use or setup, but if you have half a brain you will be creating banners in no time. As far as I can tell so far there is nothing I can't do with it and if you do try it you could msg me or post back here and I will walk you through some of functions. You can get it for free as open source at The Gimp - Image Manipulation