I would not focus on controlling the size of the images client-side. When I code anything "admin" or "cms" that includes an image I usually handle the rest server side.
All these websites you see with a huge collection of products didnt require the admin user to size the product images right before uploading them to the website. If it did m it would be very badly coded.
To accomplish what you need the only question I had was: How does flash upload files to a server? and on my first search in google the first link was this;
Upload with Flash 8

great!
In reading this bit;
Flash can't upload the file by itself, and instead needs a back-end language to do the server side job.
It sort-of confirmed my theory about needing another layer, so I went on to click the "continue reading" link... if you go to page two (skipping the flash stuff) you can see that they use 2 simple lines of PHP to handle moving this image;
move_uploaded_file($_FILES['Filedata']['tmp_name'], "./files/".$_FILES['Filedata']['name']);
chmod("./files/".$_FILES['Filedata']['name'], 0777);
and line 2 is not even needed! you literally need the one line (as well as the flash code of course).
So to extend this, all you would need to do is write a little PHP that copies this file to as many directories as you need sizes of the image, while resizing and scaling on the fly.
ie. if you had
/images/uploaded/
/images/thumbs/
/images/blog/
you could (psudeo code)
move_uploaded_file(params here, "http://www.designforums.co.uk/images/uploaded/") // this moves it to the main "uploaded" directory.
then have more code that copies this to the other two directories in whatever scale/target size you specify.
copy from "uploaded" to thumbs, target scale 100 * 100;
copy from "uploaded" to blog, target scale 200 * 200