What does "image manipulation mean?
It means modifying images in some way from their original appearances. It is also known as "image editing" and "photo editing." You can probably find ten of thousands of instructional pages on the web, as well as tens of thousands of hours of videos, all showing you how to manipulate images. The few pages I have here are related solely to fixing images of certificates that come out of your scanner, electronic camera or smartphone.
I compiled the tutorials below using two different image editors: Adobe Photoshop and GIMP. Photoshop is the leader in commercial software and GIMP is the leader in free programs. Although they approach problems a bit differently, their basic capabilities are quite similar. Photoshop has been around since 1987 and GIMP since 1995. Neither are newcomers. Although Photoshop has a larger user base, GIMP is no slouch. Both have lots of proponents who have prepared excellent video tutorials for all you will want to do and more.
Understand that Photoshop and GIMP have at least 50 more competitors. I do not have the time, money, need or pateince to test editing software so I cannot give specific advise on any other than Photoshop or GIMP. Most offer free trials. I beg you not to buy until you determine that a specific program fits the way you solve problems. I've been at this since personal computers were invented and can tell you from experience that if a program does not fit the way you THINK, you are not going to use it.
These tutorials are intended to help you get started. They are PDFs, so you can download them, print them out and proceed at your own pace. Most other image editors do the same things as Photoshop and GIMP but will often take different approaches. If nothing else, these tutorial will give you hints on how other programs can help you modify images of certificates.
I am neither advocating you buy any specific photo editor nor am I advising you to stay away from any. I will say that some are dramatically better than others. Similarly. some programs are much easier to use and get used to. I seriously advise you experiment with trial versions sufficiently to discover features you absolutely need. List the features you need before getting sucked into promotions of impressive capabilities that you may never use twice.
I have written tutorials only to the depth needed to aid your understanding of how photo editing programs work in general. I cannot offer further support, but ask that you search the web for instructional videos specific to the tasks you want to perform and the applications you are considering.