I would contact the makers of the plug-in for instructions on how to re-install it. There are several places it could be. Unless it's within the Application package - and that's one of the places - reinstalling won't help.
I found the following at this website: Boris Penck · iPhoto Export to Tumblr · FAQ
How to remove the plugin
Go to iPhotos's plugin folder ...
open ~/Library/Application\ Support/iPhoto/Plugins/
... and delete "iPhoto2Tumblr.iPhotoExporter"
Otherwise download and use Find Any File to search for "iphoto2bumblr". If you find it you can drag it and any other file with that in the file name to the Desktop. Then launch iPhoto and try exporting again.