If you are using BTMail on a web browser you will need to log onto your account and go to the settings which are found by clicking on your user name. Once there, click on "Mail" then "Block Images" and select the option that suits your needs then click save.
If you are using an email client/app there will be a similar setting in the email client/app that you are using.
It’s a similar situation for Android.
The setting can reduce your data usage, if it’s important. If you are on WiFi, it’s less important.....but it can be annoying, at times,