how do I add bleed to an image in adobe pro?

By “adobe dc pro” we will assume you mean Acrobat Pro DC. Adobe is a company name, not a product name! 

Bleed is something that should be accounted for in whatever software you use to layout your publication, not an after-the-fact fixup in Acrobat.

Bleed implies that content extends beyond the printed borders of your page. The PDF page size must be larger than the final page size to accommodate the bleed area. The bleed area is trimmed off after printing. How much bleed depends upon the printing process used; check with whoever is printing your book cover.

The method of adding bleed with an image already in a PDF file would be to use the edit function and carefully expand the size of the image to extend into the bleed area.

If your PDF file isn't already big enough to accommodate the bleed area, you could conceivably use the Print Production Set Page Boxes tool to increase the page size for this. However, once you have to jump through these fiery hoops, I would really recommend that you go back into whatever software you used to create the cover and make the adjustments there!

              - Dov