When LimeSurvey is installed in a subfolder (e.g. https://example.com/limesurvey/), pictures uploaded in the new editor (e.g. for the List image select (Radio) / Image select multiple choice themes: click or drag a picture onto an answer option) are saved with a root-relative path such as /upload/surveys/<sid>/images/picture.png. The browser resolves this to https://example.com/upload/... instead of https://example.com/limesurvey/upload/..., so the picture is broken in the editor preview and in the survey.

The REST endpoint v1/file-upload-survey-image returns both uploaded.filePath (relative to the LimeSurvey root, without the base URL: FileUploadService::convertFullIntoRelativePath()) and uploaded.fileUrl (with App()->getBaseUrl(true)). The stored path is the one without the base URL.

On an installation in the web root the problem doesn't show, because the root-relative path happens to be correct.