"Image" - what kind of image?
If vector paths, use Edit > Edit colors > Recolor art
1.) Open your document and select the colored object (or all objects of the same color) you wish to convert
2.) Go to “Window” then select “Color Guide”
3.) In your Color Guide box, select the icon seen in the bottom left corner.
4.) Scroll down to “Color Books” and in the pull out menu select “Pantone solid coated”. You will notice that all the colors in the Color Guide box have switched to Pantone colors.
5.) With your object still selected, choose the color that is below the black arrow.
And just like that, you converted your color to Pantone and added it to your swatches.
Tip: If you want to switch another color to Pantone, simply select the object you wish to convert and push the button to the left of the Color Guide’s drop down menu.
No vector graphic = no automatic conversion in Illustrator. And depending on how this artwork is made, it might also be impossible to convert it with image editing software.
But even if it were a vector graphic: When converting automatically, it will be converted to at least 6 spot colors (in addition to any other color already used in the artwork). Not sure if you really want that.
This is definitely a job for Photoshop.
But be aware that you will most likely not get good results without using several colours.
Converting to greyscale and then monotone will not give you a satisfactory outcome.