The only way took can get hit for over optimization is if you spam the alt text. The alt text attribute should be on every image. Not all attributes get text. If the image is decorative, you leave it blank. Never put "image" or "photo of" in the text. Very briefly describe what's I'm the image. If the image is words and the words aren't in the caption, as those to the alt text. If the image is say of an apple, you just write apple or red apple. If the image is linked, the alt text acts as regular anchor text. You can learn all about it here: webaim. org