Edit: Right. You already suggested creating it as a block. 

Regarding #2. You don't need the web.config setting. But you need to enable the plugin, either in admin mode for all TinyMCE settings or from code.

[TinyMCEPluginNonVisual(AlwaysEnabled = true,
  PlugInName = "advimage",
  DisplayName = "Init options",
  Description = "Init options for the HTML editor",
  ServerSideOnly = true, // Otherwise Tiny tries to load a plugin js file as well
  EditorInitConfigurationOptions = @"{
        paste_auto_cleanup_on_paste : true,
        theme_advanced_styles : ""Image left=img-left;Image right=img-right;"",
        theme_advanced_resizing : true }")] 
 
 
  public class TinyMceSettings
  {
  }

And then some settings, please note IsDefault = true and NonVisualPlugins.

/// <summary>
/// The default settings for TinyMCE
/// </summary>
[ServiceConfiguration(ServiceType = typeof(PropertySettings))]
public class DefaultTinyMceSettings : PropertySettings<TinyMCESettings>
{
    /// <summary>
    ///     Initializes a new instance of the <see cref="DefaultTinyMceSettings" /> class.
    /// </summary>
    public DefaultTinyMceSettings()
    {
        this.IsDefault = true;
        this.DisplayName = "Default settings";
    }

    /// <summary>
    ///     Gets the identifier for this specific settings.
    /// </summary>
    /// <value>
    ///     The identifier.
    /// </value>
    public override Guid ID
    {
        get
        {
            return new Guid("9d776ac1-4dc7-4789-ac9c-124f34b8c275");
        }
    }

    /// <summary>
    ///     Gets the property settings.
    /// </summary>
    /// <returns>The property settings.</returns>
    public override TinyMCESettings GetPropertySettings()
    {
        return new TinyMCESettings
        {
            ContentCss = Constants.EditorCssPath,
            Width = 580,
            Height = 500,
            NonVisualPlugins = new[] { "advimage" },
            ToolbarRows = new List<ToolbarRow>
            {
                new ToolbarRow(new[]
                {
                    TinyMCEButtons.Bold,
                    TinyMCEButtons.Separator,
                    TinyMCEButtons.NumericList,
                    TinyMCEButtons.BulletedList,
                    TinyMCEButtons.Outdent,
                    TinyMCEButtons.Indent,
                    TinyMCEButtons.EPiServerQuote,
                    TinyMCEButtons.Separator,
                    TinyMCEButtons.Undo,
                    TinyMCEButtons.Redo,
                    TinyMCEButtons.Separator,
                    TinyMCEButtons.EPiServerLink,
                    TinyMCEButtons.Unlink,
                    TinyMCEButtons.Anchor,
                    TinyMCEButtons.Separator,
                    TinyMCEButtons.Image,
                    TinyMCEButtons.EPiServerImageEditor,
                    TinyMCEButtons.Separator,
                    TinyMCEButtons.PasteText,
                    TinyMCEButtons.CleanUp,
                    TinyMCEButtons.RemoveFormat,
                    TinyMCEButtons.Separator,
                    TinyMCEButtons.Code,
                    TinyMCEButtons.Fullscreen
                }),
                new ToolbarRow(new[]
                {
                    TinyMCEButtons.FormatSelect,
                    TinyMCEButtons.StyleSelect,
                    TinyMCEButtons.TableButtons.Table,
                    TinyMCEButtons.TableButtons.RowProperties,
                    TinyMCEButtons.TableButtons.CellProperties,
                    TinyMCEButtons.Separator,
                    TinyMCEButtons.TableButtons.InsertRowBefore,
                    TinyMCEButtons.TableButtons.InsertRowAfter,
                    TinyMCEButtons.TableButtons.DeleteRow,
                    TinyMCEButtons.Separator,
                    TinyMCEButtons.TableButtons.InsertColumnBefore,
                    TinyMCEButtons.TableButtons.InsertColumnsAfter,
                    TinyMCEButtons.TableButtons.DeleteColumns,
                    TinyMCEButtons.Separator,
                    TinyMCEButtons.TableButtons.SplitCells,
                    TinyMCEButtons.TableButtons.MergeCells
                })
            }
        };
    }
}

You can also create different settings and then specify them per property:

[PropertySettings(typeof(AMoreLockedDownTinyMceSettings))]
[Display]
public virtual XhtmlString MainIntro { get; set; }

#3 is more tricky. If you use a block, then you can't float it, because blocks are always rendered as block elements and not inline in TinyMCE. Sometimes we add a token in the image's description field. E.g. the editor can add an alt text like this "This is the alt text caption=This will be the image caption". Then we render what's after caption= as a caption on the image. This can be done quite easy by adding your own display template for XthmlString and then use some regex to find images that has an alt text with "caption=" in them.

Thank you!

I went for the solution to create an image with text-block and drag it into the xhtml field. It works fine. The only problem is if I try to move it to another place within the xhtml field, everything gets messed up. 

Do I have to create a ui descriptor or something?