This wouldn't be too difficult to do with BlazorForum. I haven't made it a standard feature, due to the level of moderation that would be required to make sure people aren't uploading images they shouldn't. All that would need done is to build a "Custom Build" of CKEditor (Classic Editor), that allows image uploads, and replace the ckeditor.js file that's in the wwwroot/scripts/js/ folder.
I wrote a blog post a little while back on how to create a custom build at: https://elliottbrand.com/blog/2/Creating-a-Custom-Build-for-CKEditor-5
Note: In the overview I also mention a link to an online builder tool that CKEditor provides to create custom builds. You can try that and maybe you won't need to go through the whole process I wrote about, but I haven't had much luck getting the builds created through that automated tool to work.