Do you get a lot of organic traffic to your image pages with urls including attachment_id
Search this line in Google:
that's quite a lot of indexed pages you'll be loosing if you 301 redirect them to the main posts.
That feature tells Google to not index these pages, but pass the link benefit that could be lost through the links to these pages back to the original posts. This is a fixing a mistake in design type feature, if you didn't want the images loaded individually there's a setting on the media upload page when you add the image to a post (looks like you left it as default) and this recovers most of the lost link benefit from the design flaw.
Before using this feature check with Google analytics/webmastertools or your log files if a lot of traffic hits those pages, if they do don't use the feature sine you'll loose the traffic.
This would be a feature I'd never need to use because I never link to images the way you are (hard to add text to those pages and the output isn't ideal), but if it's getting you traffic it's apparently working for you so I'd keep it that way and maybe looking into improving the output of your attachments. If you compare these two pages
SEO wise this one isn't too bad, when you uploaded the image you hyphenated the filename and edited the title attribute text, but either missed the alt text or your theme doesn't add alt text to the output. The title attribute text (that's title="words here" (this is actually the title tag since it's an attribute, what most people call the title tag is the title Element)) is ignored by Google for ranking purposes, though WordPress does use it for generating the title element (what most call the title tag: gets confusing when SEO's don't name these elements and attributes the right way) for these pages, so is important to add a relevant title attribute when adding images. The alt text is what Google uses to rank text from images and looks like your theme doesn't add the alt text. Google will be using the image filename which is hyphenated, so that's good since Google treats the hyphens as spaces.
Not so good, no image loaded, totally pointless page.