Jisho

doing something that is "勝手" means that you do it without considering proper logic or other people's point of view, and you do it to your own convenience.

in this example "勝手な想像”: your imagination (probably a conclusion or speculation) are not made by careful examination of evidence or logic but it are made so that it fits your point of view (hence, convenience).

for example: when you hate a celebrity, you tends to think everything they do is to make more money and draw attention ==> 勝手な想像

Thank you so very much for the explanation!