So there’s actually an easy way to crosspost your Tumblr posts to LiveJournal. It’s not automatic, but it’s not complicated either.
The first step is to copy the URL for the RSS feed of your Tumblog. Then, visit http://www.livejournal.com/syn to create a new LJ syndicated feed of your Tumblr. Note that creation of syndicated feeds is a paid account feature, but if you don’t have a paid account you can ask anyone who does to create the feed for you.
You’ll probably then be given the opportunity to name the new feed. If instead you go directly to the add feed page, that means that somebody else created the feed already. That person should be listed on the new-style profile page for the feed, so maybe get to know them.
Now take a look at the syndication page for your new feed. You’ll have to wait a little while before LJ grabs all of your Tumblr entries, but once it does everything you’ve posted or reblogged in the last couple of weeks will be listed here. You should only have to wait an hour or so, but if a whole day goes by without any of your Tumblr posts showing up, open up a Support request to ask about it.
When your entries show up, pick one from the list and visit its LJ home.
Once there you should click on the Share button at the top of the post, and then click on LiveJournal. If you’ve friended your own Tumblr feed, you should see a Share link when viewing your friends page.
After you’ve clicked on Share a new tab should open up with the Update Journal page, pre-filled with your Tumblr post. What’s pretty neat is that this keeps photosets together, keeps quote pyramids at the bottom of the post together, and properly links everything just as it did on Tumblr. On the downside, YouTube videos and sound files won’t work when you Share them inside of LJ.
Before you post your entry on LJ, you can delete the very first line, the one that says where on LJ you shared it from. Syndicated entries on LJ only stick around for a couple of weeks before they evaporate, so there’s not a lot of point to keeping that information. It’s also not necessary to get rid of it, I just think it’s tidier.