First you have to build some concept, how you want this toys to work - because you have to find toy with similar functions/options too edit. For example - it's good to use some ball or stick as base for hunting toys.
It can be hard to make flyball box, because you can use toys which allow dog to be on them (like Nova's agility table or Circus chairs in Petz 5) or hide in them (like dog beds, wicker basket), so you have to be creative there.
What is your problem with background? Are you using pictures with transparent background?
You can have transparent background only on .png images and I personally achieve it by erasing background by magic eraser in photoshop, but there are also ways to do this in free programs like gimp for example. I wasn't making toys for ages...
Do you have any pics, how this toy with background problem looks now? And maybe separate pic you imported to Tinker?