Inserts an image into the text buffer at iter. The image will be counted as one character in character counts, and when obtaining the buffer contents as a string, will be represented by the Unicode “object replacement character” 0xFFFC. Note that the “slice” variants for obtaining portions of the buffer as a string include this character for pixbufs, but the “text” variants do not. e.g. see TextBuffer.getSlice and TextBuffer.getText.