TwitPic has an API, so I rolled my own simple uploader with cygwin and curl. There’s no doubt in my mind that I’ve reinvented the wheel here…

Standard disclaimer applies: this is quick n’ dirty(tm):

#!/bin/bash

# curl executable.  I'm a PC.
CURL=C:/cygwin/bin/curl

# The upload script that will accept the data
UPLOAD_URL=http://twitpic.com/api/uploadAndPost

# Snag stuff off the command line
TWITTER_USER=$1
TWITTER_PW=$2
PHOTO=`cygpath -m "$3"`
TWEET_MSG=$4

# Prompt for message if it's not given
if [ "$TWEET_MSG" = "" ]; then
   read -p "Tweet msg: " TWEET_MSG
fi

# Go
echo Uploading...
$CURL \
  --form username=$TWITTER_USER \
  --form password=$TWITTER_PW \
  --form media=@"$PHOTO" \
  --form message="$TWEET_MSG" $UPLOAD_URL

# Wait for user to read result message
echo
read -p "Press <enter> to continue" bogus

Create a desktop shortcut with uid and password:

C:\cygwin\bin\sh.exe ~/twitpic.sh --userid-- --password--

Now just drag and drop the photo onto the desktop shortcut.