Besides the toothpick method, what are some other ways to transfer a picture onto a cake?

You can use piping gel. In the Wilton course 1, they show you how to do. You might check out Wilton web site. I have not tried it myself. And if you are in the mood to spend some money, KopyKake has a projector that you can transfer on to a cake. Good luck.

PS -- I found the link
http://www.wilton.com/decorating/basic/patterns.cfm

lay wax paper over the picture in reverse, then trace with a toothpick or small paintbrush dipped in piping gel. You can tint it lightly with a color to help it stand out more. Then flip over and lay on cake to transfer gel to cake. This was what they taught us in the Wilton I course.

Before I learned about buttercream transfers -- I once layed wax paper over a picture - used a pin to poke tiny holes along all of the lines. Lay the wax paper on top of your cake - use a new "footie" nylon sock filled with cocoa power - gently rub over wax paper - leaves the dot pattern on the cake in cocoa.

Also have used royal icing to create a pattern on wax paper, be sure it's very dry and hard - and used it to press an indention of the pattern into the cake.

Hope this helps.