The OOTB image web part in SharePoint sometimes fails when trying to upload an image. The web part is using the "/_api/sitepages/addimage" -api and all provided params seems to be right. We have concluded that SharePoint are not able to create a page folder in site assets due to improper characters in the page file name. We have seen this behavior when the file name contains charcters like "," or "&". Changing the file name and removing "special" charcters fixes the problem.