In reply to:

acrophony, noun.

the use of a picture of some object to represent the initial sound, letter, or syllable in the name of that object. Using the picture of an ox, called aleph in Phoenician, to represent the sound a, is an example of acrophony.

Fascinating list of words today, wwh. So, when we show the standard picture of an apple for "a", that use of the picture is an acrophony--or is the picture itself the acrophony? At least, that is, as long as the picture doesn't have script below that reads:

"This is not an apple."

Edit: Well, I'm incorrect. Apple for 'a' is not acrophony, according to Quinion. This is worth pasting:

"We don’t do this in English, as the names of our letters of the alphabet are just invented words that convey the sound indicated by the letter. But in some languages the names of letters are words that have a meaning of their own. The best-known cases are Classical Greek and Hebrew. In Hebrew, for example, the first four letters of the alphabet are aleph, which is also the Hebrew word for ox, beth, “house”, gimel, “camel”, and daleth, “door”. It’s as though our children’s alphabet, A for Apple, B for Ball, C for Cat, were transformed into the actual names for the letters, so that A wouldn’t be called or said ay, but apple. It used to be thought this wasn’t a coincidence, that the letters of the Hebrew alphabet had evolved from hieroglyphs that pictured the objects; as the hieroglyphs had evolved into letters, the names had been carried over with them. This is now not thought to be the case. The word combines the Greek prefix acro– meaning “uppermost; head”, with –phony, “sound”, hence “the sound of the initial letter”."