Knifed seems to be limited to knives and daggers, but essentially indicates stabbing (present or past tense). Renaming it to "stabbed" could allow it to apply to other bladed weapons.
Stabbing would then need cleanup. Stabbed is past tense, which fits a number of the images I've seen. Stabbing is present tense, and should be limited to instances of the act actually being carried out in the image.
Stabbing is also the gerund (verb-as-noun) form, which works for both present and past tense. "There is a stabbing depicted in this image."
The -ing form (either as present continuous or gerund) tends to be the more common form for verbs here.
Where the -ed form is used, it's usually as a participial adjective (verb-as-adjective) as in excited, embarrassed, translated, etc. rather than past tense.
I suppose you could use that sense of "stabbed" as a tag, as in "There is a stabbed character depicted in this image." It doesn't sound too natural though.
Given the choice between the two, I'd prefer we use stabbing. As for knifed, I would probably suggest we alias it (and perhaps knifing) to stabbing. There isn't enough semantic difference between a knifing and a stabbing to keep the distinction, I don't think.
knifed was a replacement tag for pool #2314, covering comical depictions of knives sticking out of the body, something the pool used to do with its "Meiling gets a knife in her because she made Sakuya mad" shtick. It's not the same thing as stabbing, which portrays it much more seriously.