You can use the tftp on the wcs if you want, just have to make sure there are no firewall blocking tftp or any acls denying traffic from the ap to the wcs. I always use a seperate tftp on my laptop, so I can see if the ap started uploading the file or not. I also wouldn't use the wcs to do the ios to lwapp conversion, since that has alot of issues. Use the lwapp upgrade tool!