How to resize an image in a rich text box

The RichTextBox control does not have built-in functionality for resizing images. You have 2 options available, resize the image before loading it to the control or use a workaround by extracting the image from the RichTextBox, resizing it, and then replacing the original image with the resized one. If you know the resize height and width, I will suggest to use the first option -

Private Sub btnImage_Click(sender As Object, e As EventArgs) Handles btnImage.Click
    OpenFileDialog1.Title = "RTE - Insert Image File"
    OpenFileDialog1.DefaultExt = "TextBox1"
    OpenFileDialog1.Filter = "Bitmap Files|*.bmp|JPEG Files|*.jpg|GIF Files|*.gif|PNG Files|*.png"
    OpenFileDialog1.FilterIndex = 1
    OpenFileDialog1.ShowDialog()

    If OpenFileDialog1.FileName = "" Then Exit Sub

    Try
        Dim strImagePath As String = OpenFileDialog1.FileName
        Dim img As Image = Image.FromFile(strImagePath)

        ' Resize your image
        Dim newWidth As Integer = 200 ' Set your width
        Dim newHeight As Integer = CInt(img.Height * (newWidth / img.Width)) ' Calculate the proportional height
        Dim resizedImage As New Bitmap(img, newWidth, newHeight)

        ' Copy the resized image to clipboard
        Clipboard.SetImage(resizedImage)

        ' Check if the RichTextBox can accept images
        If Me.TextBox1.CanPaste(DataFormats.GetFormat(DataFormats.Bitmap)) Then
            Me.TextBox1.Paste()
        End If
    Catch ex As Exception
        MessageBox.Show("Unable to insert image format selected.", "RTE - Paste", MessageBoxButtons.OK, MessageBoxIcon.Error)
    End Try
End Sub

The workaround -

Private Sub ResizeImageInRichTextBox()
    ' Get the currently loaded image in the RichTextBox
    Dim selectedImage As Image = GetSelectedImageFromRichTextBox()

    If selectedImage Is Nothing Then
        MessageBox.Show("No image is currently selected.", "Resize Image", MessageBoxButtons.OK, MessageBoxIcon.Warning)
        Return
    End If

    ' Resize the image
    Dim newWidth As Integer = 200 ' Set your width
    Dim newHeight As Integer = CInt(selectedImage.Height * (newWidth / selectedImage.Width)) ' Calculate the height in relation to the width
    Dim resizedImage As New Bitmap(selectedImage, newWidth, newHeight)

    ' Replace the image in the RichTextBox with your resized image
    ReplaceSelectedImageInRichTextBox(resizedImage)
End Sub

Private Function GetSelectedImageFromRichTextBox() As Image
    ' Get the text range in your RichTextBox
    Dim selectedRange As TextRange = New TextRange(RichTextBox1.Selection.Start, RichTextBox1.Selection.End)

    ' Check if the range contains an image
    If selectedRange.GetPropertyValue(TextElement.FontFamilyProperty).ToString() = "EmbeddedImage" Then
        ' Get the image object from your range
        Dim imageObject As InlineUIContainer = TryCast(selectedRange.GetPropertyValue(TextElement.InlineUIContainerProperty), InlineUIContainer)
        If imageObject IsNot Nothing AndAlso imageObject.Child.GetType() = GetType(Image) Then
            Return DirectCast(imageObject.Child, Image)
        End If
    End If

    Return Nothing
End Function

Private Sub ReplaceSelectedImageInRichTextBox(newImage As Image)
    ' Remove the selected image
    RichTextBox1.Selection.Start.DeleteVisual()
    RichTextBox1.Selection.End.DeleteVisual()

    ' Create a new container with your resized image
    Dim newImageContainer As New InlineUIContainer(newImage)

    ' Insert the new container
    RichTextBox1.Selection.Insert(newImageContainer)

    ' Reset the selection to the newly inserted image
    RichTextBox1.Selection.Select(newImageContainer.ElementStart, newImageContainer.ElementEnd)
End Sub