is to convert the image to a different format. This applies in a Mac as well as in any Windows OS.
You don't say what file type this image is, but typically, images in BitMap, (.bmp), or TIFF, (.tiff), are high resolution images and so are often very large.
To reduce the size of the file, (but not the size of the image), you would need to convert it to something like JPEG, (.jpg). This will reduce the size of the file dramatically but there is a consequence. JPEG images are compressed, and that means that the compression process 'averages out' pixels with neighboring pixels. This reduces the resolution and you will see blurred effects, especially at the edges of any objects in the image.
For web pages, JPEG is fine, but for photography you may find that the lower resolution produces a less than perfect image, especially if it is printed out.
Most image editors will allow conversion, but perhaps the easiest way is to open the image in your favorite editor, then "Re-Save" it as a different file type.
You can test this of course, but I would suggest you make a copy of the image being tested, and work on the copy. That way if you make any errors you still have the original.
Hope that helps.