I've never heard someone say the term "image board" so it's the former.
This is why you use exact terms, i.e. "image board" (with quotation marks)
I tried this and then the graph started to make more sense, but then I looked into the present and it turns out "image board" might be more popular due to academic articles on imageboards(perhap
s because spellcheckers don't realize it's a noun).
Oddly the search statistics show the US predominantly uses the stranger form of the noun.https://smartech.gatech.edu/handle/1853/53595?show=full