Try this fun experiment:
1) Make a Line or XY chart using the conventional markers
2) Insert a picture (or piece of clip art) onto the worksheet and resize it
to about 1/8 "
3) Select and copy the photo, click on the data series in the chart and use
CTRL+V to paste.
All the markers have been replaced by the picture
When you ask about using Letters one wonders if you would be better with
data labels.