If you’re not used to adding text descriptions, it’s unfortunately quite easy to forget. However, help is available! If you follow the Please Caption bot at @PleaseCaption@botsin.space ⧉, it will automatically tell you if you’ve forgotten to add a text description.
Text descriptions are expected on the Fediverse, it’s seen as good manners to be considerate of screen reader users. By the time you’ve been doing it for a while, it becomes second nature.