Ask before displaying content in Gmail
There’s a way in Gmail to actually stop this content from being shown in a summary view before you allow it to.
Open the settings by click on the gears icon in the top-right hand side of the screen, and then click on options. There, you’ll see an option next to the words “external content”. You’ll want to tick the box that asks whether you want to stop content from being displayed unless it’s from a trusted sender.