Why would you want the functionality to be in a DLL, exactly?
As far as libraries that facilitate easy 2D programming, I would strongly recommend you take a look at BGRABitmap and its accompanying BGRAControls GUI component package. (Assuming you're using Lazarus, of course.) For Delphi I would say maybe Graphics32? Although personally I find Graphics32 to be remarkably user-unfriendly.... all sorts of very similarly-named functions with slightly different acronyms at the end and no explanation anywhere of what they actually mean.
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