milestone celebration
An image drawn by an artist to celebrate hitting certain milestones, such as the number of followers on Pixiv, Twitter, etc, or the number of hits on their personal website.
The number of followers is usually displayed in the background, shown with finger counting, on a sign a character is holding, or written on a character's body.
A text thanking their viewers, subscribers, followers, etc. for helping them to reach the milestone is very often part of such an image.