Not specific to images be we have a developer dashboard we use to help with housekeeping where we try to solve this problem. It focuses on using IContentModelUsage.

The code sample below is an extract of some extension methods we use - I'm not going to say it is as pretty or as well written as it could be, as it is mostly a dev tool, but hopefully it provides some guidance,

        public static IContentModelUsage ContentModelUsage()
        {
            return ServiceLocator.Current.GetInstance<IContentModelUsage>();
        }

        public static IContentSoftLinkRepository ContentSoftLinkRepository()
        {
            return ServiceLocator.Current.GetInstance<IContentSoftLinkRepository>();
        }

        public static bool IsUsed(this ContentType contentType)
        {
            var usages = ContentModelUsage().ListContentOfContentType(contentType)
                .Select(usage => usage.ContentLink.ToReferenceWithoutVersion())
                .Distinct();

            foreach (var usage in usages)
            {
                if (usage.IsUsed())
                    return true;
            }

            return false;
        }

        public static bool IsUsed(this ContentReference contentLink)
        {
            if (contentLink.IsPage())
                return true;

            var links = contentLink.GetSoftLinks();
            foreach (var link in links)
            {
                if (link.OwnerContentLink.IsUsed())
                    return true;
            }

            return false;
        }

        public static IEnumerable<SoftLink> GetSoftLinks(this ContentReference contentLink)
        {
            return ContentSoftLinkRepository().Load(contentLink, true)
                .Where(link => !ContentReference.IsNullOrEmpty(link.OwnerContentLink));
        }

        public static bool IsPage(this ContentReference contentLink)
        {
            var content = contentLink.GetContent<IContent>();
            return content.IsPage();
        }

        public static bool IsPage(this IContent content)
        {
            return content.GetOriginalType().IsSubclassOf(typeof(PageData));
        }

This is then set in our view model, of which I've included relevant parts below:

                var usages = ContentModelUsage.Service.ListContentOfContentType(contentType);

                var items = usages
                    .Select(usage => usage.ContentLink.ToReferenceWithoutVersion())
                    .Distinct()
                    .Select(contentReference =>
                        ContentRepository.Service.Get<IContent>(contentReference)).ToList();

                var modelContentType = new ContentTypeViewModel
                {
                    ID = contentType.ID,
                    DisplayName = contentType.DisplayName,
                    FullName = contentType.FullName,
                    ModelType = contentType.ModelType,
                    Base = contentType.Base.ToString(),
                    IsUsed = contentType.IsUsed()
                };

One day I might rewrite this to be a bit more pretty, in fact one day I'll probably have to when we get around to looking at CMS 12. 

Actually I think I may have gone too specific to our use case in some of that and it is probably more IContentSoftLinkRepository that you need. Our dev tools looks at if a content type is used as a whole, we then drill into individual usages. On our dev tool it actually only allows the option to delete blocks if it isn't used on a page, which then in turn allowed us to see if we could remove the content type entirely as a means to clean up a legacy project. But you should be able to use the same approach to see which images are used.

They key to that is the .IsUsed() extension method targeting the content reference.