Check your app settings to make sure it is set to automatically download images. You are probably also seeing a box at the top of the emails saying that Thunderbird is protecting your privacy by blocking images. If this is the case, do the following:
Got to Tools>Options
Click the "Advanced Tab" (gear icon)
Click "Config Editor"
You'll get an Alert box pop-up... Click the I’ll be careful, I promise! button
From the about:config menu, Find the mailnews.message_display.disable_remote_image entry. The easiest way is to type it into the filter at the top.
Once found, Double-Click the Entry until it says Status = user set, and Value = false. Once done you can exit, and changes will happen immediately.