I think what you want to do is alot like the image cropping I wrote which automaticly crops a image and limits its size upon uploading..
Although yours would only apply cropping if the image is two large.. hell you can even just put a message output saying... hay dont do that.. resize your image.. upon upload.
I just give you a quick overview of the system although mine works just like facebook using jcrop..
I use cute ajax uploader to handle my upload but you can use standard dot net I am sure.
After the upload, I then display the file to the user with a jcrop interface,
The interface then allows the user to crop it using jcrop and then it post back to the server the cords they selected in jcrop.
I then process the image myself using asp.net image handling code.
Asp.net has some nice stream writers and can easily be combined with asp.net imaging encoding to re-size a image.
A quick google search will show a lot of examples.
It was a little bit of work but well worth the control...
Im guessing though, when working with MonoX blogs, if you wanted to use your own uploader you would also have to make it work and store files just like the native MonoX uploaded.. for more help on that one you may want to ask the MonoX staff.
But my guess is for this to work, you are going to have to write your own up-loader and image handler.