A fully clothed women in a still image still has no personality, no interactions, no lines, no inherent character. A still image is within itself a prop or an object.
So criticising a still image for objectifying women is inherently both true but also completely redundant. Everything in a still image is static, it is an object, plants, animals, women, men, naked, fully clothed, they're all objects.
The term "objectifying women" (as a negative) originated from turning women into objects or props in situations where they wouldn't otherwise be so. So for example, you take a female character, you remove all of her lines, put her in sexy clothing, then have the male characters stand around and talk about how sexy she is. That's objectification, they've literally turned that women into an object or prop for male character's use.
Calling all sexual images of women "objectification" is simply an overreach. It also doesn't make rational sense when applied to women who couldn't have dialog, personalities, or interactions such as a still image on a t-shirt. It is pretty hard to have ANY women or man on a t-shirt who isn't an object.
You can call it "distasteful" (likely true) or "inappropriate" (again, likely true for work) but you didn't, you called it objectification which it objectively is not.