If a non-premium player buys a premium background with an apple, will I be able to use it on any wolf, or just the one I bought it with? Thank you ahead of time!

You will only be able to use the premium background on one wolf. If you change the wolf's background or sell the wolf, you will have to pay another apple to get the premium background back. This applies to both basic and premium members.

Whoa, OK! Need to be careful with the wolf I put one one then! Thank you for sorting that out for me. Have a good day!