Is it good to submit a sitemap for images? Which plugin should I use in order to automatically ping search engines to index new images? I see hundreds of my blog images already submitted in Google btw.
I can be mistaken, but your images are submitted in Google automatically after some time has passed. The same as with new websites or blogs.
I'm never heard a sitemap for individual pages and I think sitemap is for individual pages of the site.
I think "a site map, sometimes written "sitemap," is an overview of the pages within a website. Site maps of smaller sites may include every page of the website".
Here is one help article from Google about image Sitemaps... It says that you cannot add images into the index manually, Google algorithm picks it automatically. Check it out here, http://support.google.com/webmasters/bin/answer.py?hl=en&answer=178636
You don't mention what platform you are using which makes it hard to recommend any plugin Software like A1 Sitemap Generator can create image sitemaps, but if you are using a CMS it may be better to use a CMS plugin/module.