Currently if you want to mail photos from Picasa 3 in Thunderbird 3 fails due to this bug .
Workaround for this problem
Open terminal and run the following commands
$ cd /usr/bin
$gksudo gedit picasa-hook-email.sh
Copy and paste the script below into the editor, and edit (if neccessary) the "PATH_TO_TBIRD=" line to the command that normally launches Thunderbird. (If you have a desktop launcher for Thunderbird then open it with kwrite and look at the "exec=" line.)
# Edit the next line
emailstr=$(echo "$1″|sed ‘s/mailto:?/to=,/'|\
sed ‘s/&cc=/,cc=/'|sed ‘s/&bcc=/,bcc=/'|sed "s/&body=/,body=\'/"|\
sed "s/&attach=/\',attachment=\'file:\/\//"|sed ‘s/&attach=/,file:\/\//g')"‘"
$PATH_TO_TBIRD -compose "$emailstr"
#End of picasa-hook-email.sh
Save and exit the file
Or You can download this script from here
Now you need to change the permissions to the script
$chmod 755 /usr/bin/picasa-hook-email.sh
Now you need to close your thunderbird and open it again you should be able to mail photos from Picasa 3 in Thunderbird 3
Thanks to Josh Stump for this tip