Are you using a Microsoft Windows computer or an Apple computer or device, some people sometimes have problems with posting pictures on here via Apple devices, it might be that there's some sort of incompatibility problem between the software that operates the forum, and Apple devices. If you can post an url, which you do in just the same way as if you were posting text into your message text window, then upload your picture to a picture uploading site, which you are already aware of, and then post the url of you uploaded picture into your message, and when your readers click on the picture url in your message, the picture will open and display.