I learned this from a police photographer I was having a beer with after we worked together one day. A photograph only has evidentiary value when it's led with a statement of when/where/how it was taken by the person who took it.
Therefore, if the person who sent you the image wants to testify and lead the photo as an exhibit, then it's all good. You however can't really testify to it at all.
There are some obvious exceptions to this of course, such as when you find an incriminating photo on someone's computer, and of course, child exploitation material images.
I've been called before to give evidence about whether an image located on a suspect's computer had been modified (by the police). I discussed the kinds of artifacts that indicate modification, including pixellation, location of light source, and so on. The judge was quite satisfied that the image was legit. I had done classes on multimedia in Uni, and I'd seen a lot of images on the job, and that's what I relied on to make my conclusion. Generally though when it's an image of a computer screen, I'd be a lot more comfortable saying that something WAS altered, than saying definitively that it HAD NOT been altered.
EDIT Further elaboration on what my actual evidence was in this case.
After discussing artifacts which occur from editing a photograph, I indicated that it would take a high degree of computer and image software skill to fake a photograph such that a person with my level of skill wouldn't see any artifacts under close examination.
I then discussed the level of computer skill of the specific officers against whom the suggestion of alteration was made. I'd just that morning helped them to access their email because we were at a remote location, and the procedures were different outside of HQ. They clearly weren't very tech savvy.
The conclusion that I drew, that was accepted by the judge, was that these officers didn't have the level of technical skill required to alter these photos such that it wouldn't be obvious to someone like me.
For the next 4 years until I left QPS, these 2 officers took great delight in telling other police how I called them stupid on the stand, which I didn't, but it was was a bit of fun for the 3 of us.