ImageMagick: Handling Raster Images (Open-Source, Multiplatform)
ImageMagick is a free and open-source cross-platform software suite for displaying, creating, converting, modifying, and editing raster images, such as photographs.
On Linux, ImageMagick looks pretty bare-bones.
To open the menu, click on the app window.
You will want first to open a file: File > Open
The ImageMagick suits includes a command-line utility (convert) for converting between image formats and such.
convert Between Image Formats inter alia (Command Line)*
Extracting the Coordinates of a Point
To learn the coordinates of a point, start the GUI for ImageMagick, then click the sequence Image Edit >Draw, [remove the menu subsindow by clicking the [x] sign,] and the coordinates of your pointer will be shown in the top left-hand corner as integers.
ImageMagick in Linux
In my Lubuntu Linux, the graphic interface is named display-im6.q16, as explained in catch-all by man 1 ImageMagick-im6.q16. So you would type display-im6.q16 IMAGE.EXT at the command line, and then whatever, such as extracting the coordinates of a point in the picture.