To begin with, it looks like you are trying to insert a picture into a standard textbox and this code will not work for that. You will need a rich text box (Right Click on toolbox>Components>Microsoft Rich Textbox control>ok).
Dim P As stdPicture
P = LoadPicture(StringVariableThatContainsThePathFileNameToAPictureFIle)
'or P = LoadPicture("PathFileNameToPictureFile")
InsertPicture RTB, P
Did you dimension P first as a stdPicture object?
Are the first four references under Project>References the following?
Visual Basic For Applications
Visual Basic runtime objects and procedures
Visual Basic objects and procedures