If you try to enter an image in your post, which is larger than the forums maximum size (1024x768 pixels), you will get an error message telling you that your picture is not acceptable. Each picture that you try to include in your messages should be smaller than the maximum size.
Also, please note that your pictures should be accessible from the Internet. If one of your pictures is not accessible (for example, because you entered its URL wrong), then you will also get an error message. You need to delete the link to that picture, or replace it with the proper link, if you want to continue. Some browsers, like Internet Explorer, will show you a placeholder for the non-existent image. Click on it and delete it, then enter the proper URL enclosed in img and /img tages. For those browsers which do not show you a placeholder for the non-existant image, you have to delete the whole area where you typed your URL and replace it with the proper URL within img and /img tags.