Thank you for your input. Have you noticed the GitHub link in the top right corner? I believe this provides a clear and user-friendly way to give credit.
> What is Small.im? Small.im is an open-source batch image compression tool that supports multiple image formats.
And at the top you put a GitHub link to the original project. If someone doesn’t click on that link and do a bit of research, they might think you are the author of the open-source project, when in reality you just deployed your own fork without publishing your source and without mentioning the parent project (and with "© webmaster.studio. All rights reserved." at the bottom).