I still gotta get confirmation of this, I keep getting contradictory answers. Does it not compress any photos or videos, or just photos?
Pretty sure it was a bullet point when they introduced it.
Rewatching the 2011 iOS 5 Keynote, Scott Forstall refers to "high quality" images. (Basically throwing shade at SMS/MMS which is known for compressing and shrinking images to hell)
Do a test. Have someone send you an image that they take with their phone over iMessage and then have them send you the same one through AirDrop and download them to your Mac (if you have one) and compare them.
Actually, I did a test of my own:
I took a photo with my phone and opened Image Capture to pull it to my computer to get the original photo. This one was named IMG_0909.jpg
I first send it through AirDrop to my computer. It comes in as IMG_0909.jpg.
I then sent the photo to myself and saved the photo to my library after deleting the original and opened Image Capture to pull that file to my computer. This one was named IMG_0910.jpg
Then I compare all three. The one that was sent through iMessage is slightly larger. But I dont think it changed the pixels at all. I think it might have added a few KB of metadata or something. I open them all in Preview in one page and switch between them and I don't see a difference. Not even a slight compression or anything. Not a pixel changes. The resolution is still the same. The EXIF metadata is only slightly different with the one from AirDrop including the name of my phone in the "Where from" section. All three files report as 238K but only the iMessage one has more bytes. (233,775 bytes vs. 236,298 bytes) And they're all original resolution of 960x1280 (FaceTime camera) So I don't know what these extra bytes are, but I don't believe it's doing anything to the image data itself. I don't have the proper tools to compare the two images on a pixel by pixel basis, maybe you do? Either way, it looks to keep the original in tact completely from what I can tell.
Maybe I'll try again with the rear camera instead since that's probably bigger.
Edit: Yeah. Same results. Used the rear camera. All three images are 2448 × 3264, all three are 2MB with the iMessage one being 1,983,926 bytes vs. 1,979,830 bytes of the other two. Why it's 4,096 bytes (4KB) bigger is beyond me. (The other image was 2,523 bytes bigger)