An image font is created by having a series of characters set next to each other in a horizontal row. Between all the characters and on the left and right sides should be a dividing line consisting of a single color (which is not found in the actual characters themselves).
This example image is consiting of black and white glyphs on a pink background (background can be transparent) where they are separated by yellow lines. The yellow will be used in determining where each character begins and ends.