The below discussion is for Excel 2013. If a cell is selected and it has comments, the comments will be displayed. Sometimes, there may be a need to display picture when you select a cell. I answered this question on Microsoft Community Forum and I have decided to reproduce it here.
1. Right click the cell > Edit Comment
2. Now, you need to right click on the border of the comments box. Do not select the comments inside the box and choose Format Comment. Since, you are inserting picture, hence you should not have comments in the box, anyway. So, there is no question of selecting comments inside the box. Once again, I will repeat that RIGHT CLICK ON COMMENTS BOX BORDER AND SELECT FORMAT COMMENTS.
3. Now, you will get Format Comments box and you need to select Color and Lines tab.
4. Click on Color: box and select Fill Effects.
5. When you click Fill Effects, choose Picture tab and select picture from there. By default, it starts connecting online to select picture. Most probably, you would like to select a picture on your computer. You may like to choose Work Offline here.
6. Now, you can have effect of picture in your comments like below –
