didnt upload the picture

Thunderbird does not modify the subject line. Use your best judgement as to what to do with this message.

How do you remove it? Delete the message.

I would like thunderbird to not display any icon in the future.

Look at the message source (ctrl+u). I suspect you'll find some UTF-8 encoding that summons the image from a font.

Here the subject line from a message from booking.com that shows a picture of a calendar:

Subject: =?UTF-8?Q?=F0=9F=93=85_Chris,_is_it_time_for_your_next_trip=3F?=

The available images can be seen here:

http://www.utf8-chartable.de/unicode-utf8-table.pl?start=128000

Seikusa said

I would like thunderbird to not display any icon in the future.

Thunderbird displays what you receive. As stated earlier, Thunderbird does not modify subject lines. This is not a Thunderbird issue.

so is there a way to forbid utf8 display in subject lines, maybe via addons?

They are not icons. They are glyphs, characters if you will, in an extended character set that embraces a large number of written human languages. They are as legitimate as the regular letters of the alphabet. As such they are not a security risk.

Please read up on unicode and utf-8. These images are part of the subject line. They are not inserted or downloaded from anywhere.

Did you look at your subject line in the message source as I suggested?

I did and I found the UTF8 hex values. Didnt knew the ctrl+u shortcut. Still I would like to not display them. Is there a way to filter them out maybe through regular expression with the use of an addon?

The whole point of Unicode and utf-8 is that people can use their own language in email. Why should those who speak non-western languages be forced to use our Latin alphabet for their subject line?

So Unicode and utf-8 make email more accessible to more people. It happens to include some childish icons. What is there to object to?

Complain to those who write to you using these images. It's their choice to use/abuse/misuse the available technology in this way. Let them know it leads you to distrust their missives.

Thunderbird is merely showing you what it recieved.

I like Thunderbird for being simple looking & clean, therefore "popping" utf8 characters are something not appriciated. Thank you for answering. I will try to find a solution through addons and post any findings here.