Please update the image in your Google+ profile (I believe Gmail web also has its own photo, but it's not always in sync with G+).
AquaMail fetches profile images from G+, believe that's the only supported / official API (source, function).
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After updating your G+ profile, long press the account in Aqua -> Account setup -> Next, Save and next time you check mail in this account, Aqua will request your image from Google again.