What should I do when images in a publication appear to have been faked?
Manual alteration of figures aimed at deceiving readers (by fabricating, obfuscating, 'cherry picking' results, etc.) is a serious matter. It surprises me that people still try because it's very often quite easy to spot (other types of data fabrication are harder to catch). But it's sadly not uncommon, even in highly regarded journals and from researchers from reputable institutions. 

For life sciences, according to one of pubpeer's moderators in a comment: 'Most of the life science reports involve image manipulation - a good majority are gels, with a bunch of duplicated specimen images as well. […] we see sometimes on PubPeer […] things like doctored NMR spectra in chemistry.' The latest seems to be related to your observation, although it's outside my area of expertise.

You can get a sense of the type of things that are reported on PubPeer reading this thread among many others. As an example, a close examination of this figure shows the use of copy paste to fabricate data: 



Or this one, initially published, not in your average pay-for-publish shady 'open access' journal, but in Nature, that has a rather obvious copy-pasted middle panel:

 

(both the articles where retracted).

In your position, your first reaction is totally appropriate. Here is what I think is the best course of action:


Contact the author(s) it is a good way of showing that you are concerned but not necessarily interested in public shaming. If the authors do not react, then: 
Contact the publisher (mentioning that you already contacted the authors to no avail) as suggested by @alarge in a comment. If it's a reputable publisher, the issue will be taken very seriously.
If all fails, you are left with public reporting of the issue, anonymously or not, via social media or the website listed above. Note that you are always at risk of putting yourself in trouble when reporting misconduct, the same as in any other field, so weight this risk if you intend to associate your name with the complaint.


EDIT:


  Should I report that to the journal or leave it as it is (may ruin
  someone's career)?


I think that, as a scientist, you have a responsibility to report that sort of misconduct when you see it. What may ruin someone's career is their sloppy ethics, not your concern for integrity.