In general, using images in your graph should be avoided. Similar to using a bar graph with a vertical axis starting above zero, using images in your graph can mislead an audience. An image is generally perceived as having volume. Increasing the image size to meet the y-axis value desired, is visually increases the image size along 3 dimensions.
For example, the graph above shows a graph featuring a tent to represent the number of tents bought by the non-profit organization from funds donated during two particular campaigns. Though the Campaign B only surpassed the first by 10 tents, it visually appears to have increased by 30, because the image was increased on 3 axis.
If you really love images and want to use them in a graph, consider either putting the image within a bar on a bar chart or use the image as a point on your line graph, as seen in the graph below.