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In particular I was not able to find 図象 in any dictionary; I wonder if it is perhaps a typo and the author meant 図像.
In the case that the author meant 図像, would it then simply mean "icon made up of characters"? So that 亜字形図像 could be roughly translated as "the 亜 (character-based) icon"?
The context is the following:
中国古代の有力氏族には、亜の職（亜職）についている人がおり、その人は「亞」の形の中にその氏族名を記したものを紋章のように使用していた。これを亜字形図象という。
Source: 白川静；常用字解（第二版）、p３
In standard usage, 象 and 像 are distinct kanji with different meanings. In particular, they are rarely confused in the following contexts:
However, these kanji are also often associated with "image":
In such cases, 象 and 像 were not strictly distinguished in the past. For example, if you search the 青空文庫全文検索, you will find that older authors sometimes used 実象, 映象, 群象, etc., which would be considered mere typos in modern standard usage but were accepted variants at the time. In your specific case, it's safe to consider 図象 as a synonym for 図像, so it's "亜-shaped icon".
(I doubt this is a "typo". Although 常用字解 seems to be a relatively recent book, I suppose the author intentionally chose 図象 based on some historical or academic background. Old notations tend to be preserved in the context of historical researches.)
Please also read dROOOze's answere here: How does ayakashi, あやかし = 妖 or 罔像?
Not positive, but I think it's (((亜字形)図)象): that is 亜字形, plus the suffix ～図, plus another suffix ～象 (that I would say is read as ～[象]{しょう} - "phenomenon").
亜字形図象 - "The 亜-shaped icon/image phenomenon"
かなっ？
図象とはinto a search engine - the standard trick I was taught - the first result I get is a Goo entry, then a Kotobank entry, then the other word in Kotobank, then a Weblio entry, then JP Wikipedia.