"群众心目中的形象" can be translated into English as "the image in the minds of the masses" or "the public's perception of one's image".
Here are more context-specific translations:
For a more general or formal context: "the image held by the masses" or "the public's perception of [someone/something]'s image".
For a more colloquial or everyday context: "how [someone/something] is perceived by the public" or "the public's view of [someone/something]'s image".
Choose the translation that best fits the specific context and tone of your sentence.