This one is easy and no "enrolling":
1. Take a screenshot of what you want others to see; you can use the built-in Windows "Snipping Tool", Irfanview's Options/Capture Screenshot, or a zillion other apps.
2. Go to https://imgur.com/upload
3. Browse to or drag image, upload
4. When image appears on the site, you can either:
a) Right-click it and choose "Copy link location" (dialog from Firefox context menu, your browser may differ); this will give a link to the image file itself (ends in for example *.jpg). You can then use the MusicBee forum's "Insert Image" dialog button and insert the link, which will embed the image itself in the post.
b) Or, click on the Imgur "Copy" button they provide there, which will produce a no-extension link you can insert into a post using the "Insert Hyperlink" button. This provides a LINK to the image in your post instead of embedding it as in a), which is preferable if you want to use a huge image that otherwise would take up too much real estate in the forum, which can be annoying.
Be sure you check your post first using "Preview" to be sure it's what you want before hitting "Post" (note the difference between using "Insert Image" and "Insert HyperLink").
Other sites/methods abound, but as I said, this is the one that works easiest for me.