I was worried about possibly needing to change license.
I’d rather ask the contributors to consent to licensing their code under the new license. You don’t need the copyright in the hand of one entity to change license, it’s enough if all copyright holders agree.
The situation is made seemingly complicated by the possible need to use copylefted images
WDYM by “images”?
As in art assets? I’m not sure those would even be infectious. I think it’s possible to even use non-free assets in a GPL’d application. It may be better to treat them as such to keep the licensing simple though.
Even then, it’s usually possible to “upgrade” permissively licensed code (such as Apache 2.0) to a copyleft license as long as the original license’s conditions are still met which usually involves denoting which parts of the code is also available under the permissive license.