ehm actually my 4images is doing all of these things and furthermore it puts an annotation on the 800px picture!!
i just added some mods here and everything is running, of course you have to read the threads in total, because in vano's thread he mentioned above everything on resizing is written, even with saving the original file in /big/ folder!
Here you can see:
http://www.4homepages.de/forum/index.php?topic=7700.msg22313#msg22313If you now combine the two mods called here, you should come to your result!
for me its working very good, i can use one of my images, upload it, its getting resized, placed a watermark on it, make thumbnail and save the original file to /big/ folder.
and now by clicking on download i get my big original picture!
the only thing you have to do, read the forum.
i'm totally a noob in mysql and php, but i got everything i need from this forum (great thanks to all here, especially jan and vano
)