How to convert photo into pantone for spot printing?
Yes there are different methods to send a file. In all cases, a good comunication with the printer is a must.
Ryan posted one using alpha channel, this is a transparent image.
Another option is to simply send a grayscale image and talking to the printer that you want a spot ink there, other than black. (Image > Mode > Grayscale)
A more advanced option is after making one grayscale image and after that now make it a duotone (Image > Mode > Duotone) If you only use one ink is a monotone.
Watch the contrast
There is a chance you need to increase the contrast (and brightnes) on the photo, depending on what pantone are you choosing.
Watch the absorption on the paper
If you are reducing the costs using one ink alone, your paper probably is lower quality too, so be prepared for not having al the middle tones you expected, increasing the contrast, again, will help you a bit there.
You can convert an image directly from RGB to grayscale, but you also can try using a specific channel.
Imagine this photo: A blue sky as a background with a tree with red leafs as your main subject.
If you want to print that on a blue ink you could use the blue channel. Or if you want to make emphasis on the red tree, use a red ink and use the red channel.
You can use the channel or just before making an image grayscale you can lower some RGB values using curves.
Your best bet is probably to talk with the printer and find out how the'd want it but I imagine you'd want to provide a single Alpha Channel with the Pantone on it, some might accept it as grayscale though and just tell them what pantone to use when printing.
To make the Pantone Image for them if needed:
Open the Channel panel and Ctrl/Cmd+Click on the RGB channel so it loads as a selection:
Click on the second icon the bottom to Save Selection as Mask. You should now have an Alpha 1 channel:
Make sure everything is deselected and then while on Alpha click the channel manu on the panel and choose Channel Options. Then select Spot Color and make your pick. For what you're doing make sure its set to 100.
Then when you go to RGB you can see the result.
Depending on what you're going for you may want to invert the channel. Click the Pantone channel and hit Ctrl/Cmd+I
For what you're doing since you want the entire document to just be the pantone then now that you previewed and make sure you got the spot and inversion how you like it you'll want to save just that channel. So either Duplicate the Pantone Channel to a new Document or click the RGB channel (which will select RGB, Red, Green, and Blue) and then delete them from this one. You'll be left with:
This is why I said at the beginning some printers might be fine to just give them an Alpha channel or even a Grayscale and telling them which Pantone to print it as.