I'm currently trying to figure out how to get LJ Add-Ons to do it. I know you can use it for DW so there's no reason the same shouldn't be true for IJ.
STEP 3 Click on extensions if you're not already there. Then, click on options under Livejournal Addons 9.2.
STEP 4 This window will pop up. Click on that little letter looking thing at the top. Make sure 'randomize icons' is selected. It should work on IJ, if you check on "On all" or fill in the "External sites" box with insanejournal's url.