There is no one right way to cite sources within slides or MS Office PowerPoint presentations..
First, choose a reference style (or use the one that the professor tells you to use) and use the same one for all the references in your document.
Remember, the primary purpose of citing is to verify where you found the information so that someone else can go to the source to get more information.
Here is an example of citing a journal article image in NLM style:
Fong A, Garcia E, Gwynn L, Lisanti MP, Fazzari MJ, Li M. Expression of caveolin-1 and caveolin-2 in urothelial carcinoma of the urinary bladder correlates with tumor grade and squamous differentiation. Am J Clin Pathol. 2003 Jul;120(1):93-100. Image 4, Immunohistochemical staining of a urothelial carcinoma with squamous differentiation with anti-caveolin-1; p. 98.
NLM is similar to other numbered styles (AMA, Vancouver). These styles put a number in the the text and organize the references at the end in number order.
The other major style format is APA style. Purdue OWL has a nice guide on using APA style. This style puts author names and dates in the text and organizes the reference list in alphabetical order.
It you are putting the entire reference to the image on the slide being viewed, your choice of numbered or alphabetical doesn't matter, so you could follow the format above.