What is the proper way to cite an image source, if they were given to me by a company?
The most important function of a citation is to inform readers where they can find the information you are citing.  If the information you are using was provided for your privately, then your source is “personal communication”, meaning that the information cannot be found in any published literature, but only through personal contacts.

In this case, I would do the following:  In figure captions and/or the body text discussing the figures, make an explicit statement that you received the images from the company involved.  One statement may be sufficient (something of the form: “Figures 3–6 were provided by Prescott Pharmaceuticals,” when you are discussing figure 3).  For each figure, also provide a citation in the caption to “Prescott Pharmaceuticals, private communication” (or however private communications are formatted with the citation system you are using).