Firstly, you shouldn’t let this stop you from making a contest, as your edits are nice and there’s also lots of other prize options you can do!
Some suggestions; following on IG, promoting your account/story on IG, full story read, 3 episode beta reader/proofreader etc.
These are ones that I’ve seen and used from my time on the forums. Not everything has to be literal “artwork” to be a good prize 