Background Pony #32C7
IMO, when a character is so barely, incidentally in a cropped picture, they shouldn’t get tagged. I would remove Pinkie Pie from the tags on that image.
In other cases, that’s what the tags solo focus and offscreen character are for - although unfortunately, there isn’t a way to connect which characters are the focus and which are offscreen.
The site already sorts character tags into their own category, so simply implement a check that if someone tries to upload something with two or more character tags and the “solo” tag, return an error.
Unfortunately, this would fail (without further work to create an exception list) because some characters get multiple character-categorized tags applied in tandem: for example, golden harvest/carrot top, or sci-twi/twilight sparkle
It also fails in the case when other characters are depicted but not present - for example, a picture of Twilight Sparkle with a photo of the mane six displayed on the wall could get all of the character tags, but be a “solo” picture because only Twi is actually there.