if pictures are speech, then either CSAM is speech, or you have to justify an exception to the general rule.

CSAM is banned speech.

The point of banning real CSAM is to stop the production of it, because the production is inherently harmful. The production of AI or human generated CSAM-like images does not inherently require the harm of children, so it's fundamentally a different consideration. That's why some countries, notably Japan, allow the production of hand-drawn material that in the US would be considered CSAM.