How to correctly implement the transfer of a string from the client to the server?

Question

JavaScript

I have a React application whose one function is to generate images.
At the moment, the images are generated on the server and I can fully control the process of creating it and then sending it to the right places.
There was a need to generate an image on the client and only then send base64 to the server for further work with it. But there was one problem, I can’t figure out how to protect myself from the substitution of the final image.