As an author writing for a publication you may be asked to submit your images and charts in formats and resolutions with which you are unfamiliar. For example, the Journal Nature requires figures formatted to fit their page layout where columns are 89 mm and double columns are 183 mm. The particular demands of print means that photographic illustrations must be at a minimum of 300 dpi (Dots Per Inch).
If you open your photograph in Windows Photo Viewer or any image editing program, you can see the dimensions and resolution of the picture. In Windows Photo Viewer for example you'll find it under File>Properties. This Illustration shows a photograph 4096 x 2304 pixels at 72 dpi., which could render on a screen as big as 56.9 inches by 32 inches, however Nature wants to have it set for 300 dpi so then its size becomes 13.6 inches by 7.7 inches. That's still a shade bigger than 89 mm (3.5 inches) so we need to resize the image while keeping the 300 dpi resolution.
Here is how to do it in The Gimp (download from gimp.org) the free open source image editor.
Open your photograph and go to the Image drop-down. Select "Scale Image" and the dialog will show you the current size and resolution of the image. Change the measurement to millimeters and type in 89, then hit the "Scale" button. (If the little chain icon to the right of the values is in place, it will scale everything proportionally.)