you could also try any of the coloring book pages. i've not done a fbct, but any time i need an image for anything, i just google it.
I google images, but also find myself looking at everything around me! I use anything I can get my hands on...bought six boxes of Valentine's on clearance at WM then scanned them into my computer to resize and use later! You can use coloring pages as stated, I even find myself looking at the packages my kid's toys come in. I do resize to get what I am looking for, luckily this is a feature on my printer (will even give me the finished size in inches...before it prints!)
If you are looking for kid stuff, checkout this site. It's easy to use and free. http://www.dltk-kids.com/coloring.htm
Like the others said too, anything around the house will do too, toy boxes, party napkins, invitations. etc. 
kos
I use this website below sometimes, I think I may have heard about it from this CC. I can't remember.
www.kidscolorpages.com
They have a variety of pictures from cartoons, numbers, letters, United States, animals, Presidents, Disney, Looney Tunes and lots of other categories. You can print them or save them to your desktop, they have some options written out at the top I think. Hope this helps.
Nicole
When you find a picture on the internet try right clicking it with your mouse. Sometimes it will allow you to email it to yourself, depending on what program it is in. Once you receive the email you can open it and make changes and resize it to fit your cake. Hope you find what you're looking for.
Thanks everyone, the links are great, just the kinda stuff I'm looking for.
You guys have such great ideas, who would have thought of packaging from toys etc. thats thinking outside the box.
...we're not thinking outside the box...we're literally thinking ON THE BOX!!! 
BCT transfers are all around you, if it's flat or can be scanned, I USE IT! 
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