The Gmail message 'Images in this email are hidden' can appear for several reasons, including low sender reputation, suspicion of spam, or user/administrator settings. This negatively impacts email marketing by reducing engagement, click-through rates, and conversions. To mitigate these effects, marketers should focus on improving sender reputation through email authentication (SPF, DKIM, DMARC), maintaining clean email lists, and avoiding spam triggers. They should also optimize images by compressing them, using appropriate formats, and adding descriptive alt text. Designing emails that look good with or without images, using responsive design principles, and providing a plain text version are also recommended. A/B testing different approaches is beneficial to determine what resonates best with the audience.