When a student photo does not exist, instead of the Seating Chart in the Attendance by Photo screen showing a broken image icon (because it is, imo, malformed HTML (a bug!)) load a static placeholder image.

<img src="data:image/jpg;base64," style="width:auto; height:auto;">

Is invalid HTML, the source is not complete because it contains no actual data, eg an image of 0 length. Detect this state when building your base64 insert into the HTML and substitute in the placeholder when LEN = 0 for the data pulled from PIC.