there is a module in the forge, besides, if you want to use resize in your own module, you can use e.g. makethumbnail.ashx from UserDefinedTable module.

For what it's worth...
A common generic solution is to monitor the file upload process and/or run scheduled checks of a specific upload directory.  When a new file arrives, the file monitor executes a server-side script to run a standalone graphic resize utility on the newly uploaded file(s).  There are plenty of freebie graphic utilities that are scriptable, do an excellent job of resizing as well as converting to a specified target format (so you can allow upload of a wider range of source filetypes) but you'd need server admin access for such a solution -- highly unlikely if you're in a shared-hosting environment.
 
-mamlin