I am not a lawyer this is not legal advice.

Search for copyright assignment agreements, there are a few good template documents. I’d request a signed document instead of simply stating it in the PR. In all cases I would recommend verifying the document and process by a lawyer before you start using it.

Also, I would consider not requiring copyright assignment if at all possible for the project in question.

https://support.google.com/docs/answer/3093339?hl=en

First, thanks for working on Pharo, what an amazing environment.

I am not a lawyer so you should double check.

For GPL, indeed the image as a whole would be GPL. If the personal project is more of an “executable”, it might be what you are looking for, any further image modification would have to be released under the GPL as well.

If the project is more of a “library”, you might or might not prefer the LGPL. In this case I feel like https://www.gnu.org/licenses/lgpl-java.en.html would apply similarly to Java. As long as you can load the image and change the original library code, other code is not restricted on the license.