How to cite a rebuilt graphic
There are two parts to this question: citation and copyright issues.


Regarding citation: cite the source of the data/graphics/charts/schemes.
For the copyright part, if the presentation of the data is really identical, most journals liberally give permission for reüse (you have to ask for it), but require an acknowledgement of the form “Reprinted with permission from …”. That should cover modifications in format (bitmap to vectorial, or bitmap downsampling, etc.), but I have already seen people actually note the difference by using “Redrawn with permission from …”. Now, if the figure is based on the first one, but with substantial modifications, you can be happy with only citing the source, or possibly adding “Figure based on X with permission from …”.