What is the meaning of "もとになった"?
Here is the sentence in which I find this expression:
もとになった鳥山先生のイラストはDB大全集第6番182ページ掲載.
Maybe simply, (this illustration) originally was published on [...]?
For more context:
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the original illustration was published in ...
The version here is an altered/modified version based on the original illuatration.
Weblio translates もとになる as "become the basis". Here "もとになった" is used to modify "鳥山先生のイラスト". So it literally means:
The illustration of Mr. Toriyama that became/is the basis (of this illustration) ...
If X is the もと of Y, it means X is the original that Y was based on. So in this case, the もとになった…イラスト would be the illustration that the pictured cel was based on. (Presumably the pictured illustration is from a cel created by Toei, and the もとになったイラスト would be the illustration by Toriyama that's mentioned in the passage above.)