Here are 5 best image search engines to search for images over the web. I have excluded popular image search engines like Google Image search, Yahoo Image Search etc. from this list, so as to focus on niche image search engines.
PicSearch is one of my favorite image search engines. First of all, it is quite fast. Just type a search query, and it returns all the images that match your search almost immediately. My favorite feature of this free image search engine is that it shows the image size and image dimensions on the image results. This ensures that I immediately know which images fit the dimension that I am interested in.
It also comes with Advanced image search options, which includes searching for only Black & White images, search for images in any particular color, and search for images of specific size.
Ginipic is an image search engine that can you download and install on your computer. Ginipic lets you search for images from multiple sources, like, Picasa, Flickr, SmugMug, etc. It lets you search images from image search engines, and photo sharing websites. What I like most about Ginipic is that it can also search for images on your hard drive. So, if you already have some image with you that matches your search criteria, you can directly use that image, instead of searching for one from the web. Download Ginipic here.
Incogna is another free image search engine. There are two features in it that I like: First of all, it comes with auto complete feature. Secondly, it also lets you search for similar images. To search for similar images, just click on any image in the search result, and then Incogna will show you similar images to that image. Also check out other free software to find similar looking photos.
Gazopa is the most innovative image search engine that I have come across. It lets you search for images by uploading an image. It will then find images similar to that. You can also provide URL for an image, and Gazopa will find similar images to that. Here is my favorite part: it lets you draw an image online, and then it search for similar images. I find this a really good option if you have a rough idea about type of image that you want. It also provides conventional image search so that you can search for images by providing a search query.
TinEye is actually a reverse image search engine. In this, you start by uploading an image, or provide URL of an image. Then TinEye will search all over the web to find where is that image being used, where did it come from, and other information for the image. This is a good option if you think someone is copying your images.