Website in question is a long form content focused and will include inline images as per user input when they create their blog/ content entries hence number of images in post is not controllable.
Need to decide the suitable typeface size (as a percentage of other blog body text say 70%/60%/50% of main text if it is say 14px) for image caption. Also need to determine if italics works better than regular text for image captions.
Research is providing contradictory information with some sites suggesting to use italics sparingly, other research suggesting no impact on readability on using italics. See italics being used widely across many content heavy websites and news sites for image captions.