This should likely be clarified on the master project first: https://github.com/gothinkster/realworld/issues to specify desired behaviour across all frontends. E.g. the "reference" implementation, which is Angular-based, simply shows omits the image if not set: https://react-redux.realworld.io/ e.g. in the navbar.

The matser of this project is however definitely broken whatever the decision there might be https://github.com/gothinkster/react-redux-realworld-example-app/tree/9186292054dc37567e707602a15a0884d6bdae35

The live website is however the same as angular, does not show image if not set: https://react-redux.realworld.io/#/?_k=yix5vw But extremely confusingly it shows the default image everywhere else except login. Likely showing the default smiley would be the way to go as you propose.