What does 江信江疑 mean?
I'm basing my answer on the article 江信江疑是什么意思 which gives a reasonable explanation.
It's a play on words.  It begins with

half believing, half doubting / skeptical
将信将疑
jiāng​xìn​jiāng​yí

The line 将信将疑 comes from the poem 吊古战场文 by Tang-dynasty poet 李华.
However the meme replaces 将 (​jiāng​) with the surname 江 (Jiāng) which has the same pronunciation and is the family name of Jiang Zemin (江泽民).  Then it's added as a caption to memes where 江泽民 looks skeptical, such as:



I'm not really sure how the meme evolved beyond that to what you see on Twitter.