Step-by-Step

1. Download ImageJ from this link.
2. Unzip the downloaded file by double clicking it. The default behavior on Macintosh will be to unzip the file into a folder in the same location and with the same name as the zip file.
3. Copy the ImageJ folder into your Applications folder.
4. In the Applications folder, you will find ImageJ and ImageJ64 (for 64-bit computers).
5. When launching the program it may show a dialog box requesting Java.

Details

ImageJ can display, edit, analyze, process, save, and print 8-bit, 16-bit, and 32-bit images. It can read many image formats including TIFF, PNG, GIF, JPEG, BMP, DICOM, and FITS, as well as raw formats. ImageJ supports image stacks, a series of images that share a single window; it is multi-threaded, so time-consuming operations such as image file reading can be performed in parallel with other operations.

ImageJ can calculate area and pixel value statistics of user-defined selections and intensity thresholded objects. It can measure distances and angles. It can create density histograms and line profile plots. It supports standard image processing functions such as logical and arithmetical operations between images, contrast manipulation, convolution, Fourier analysis, sharpening, smoothing, edge detection, and median filtering.

ImageJ does geometric transformations such as scaling, rotation, and flips. The program supports any number of images simultaneously, limited only by available memory.

ImageJ runs on Linux, Mac OS X, and Windows. ImageJ and its Java source code are freely available from the National Institute of Mental Health and in the public domain. No license is required.