Some people like to stay extra secure with their inbox by not automatically downloading images included in emails, which can be a way for people to track that you've opened so you can actually make it so that if you receive emails with images, that Mailbird will not automatically download them unless you want them to.
If you want Mailbird to automatically show images in emails you can update the settings by following these steps:
1) Go to the Mailbird menu in the top left of the main window
2) Select "Settings"
3) Select the Appearance tab
4) Check or un-check the "Always show remote images" box under the header "Messages"
This allows you to always show images or to choose which images to show in each email manually.
If you leave this option un-checked you can decide on each email whether or not to show the remote image on a case by case basis by clicking "Display once" or "Always display from [insert contact]".