Mac Mail resizing (compressible?) images

Question

I have been using Mac Mail to send images for a few years now. I am fully aware of the Image Size Option in the bottom right hand corner of Mail when an image has been attached. I use this feature often. Recently, the Large and Medium settings are too small. It reduces my 4.1MB image down to 108 KB for Large and Medium, and 70K for Small. In doing  research on these forums I came across a discussion about "compressible" images. What does that mean, and what would be making them (suddenly) un- or non-compressible such that this Mail setting would not work properly?
In testing, I tried to send an un-edited image. In this case Mail works as it should, resizing the attached image reasonably, with four size options. My edited images seem to cause the problem. I edit in Photoshop (CS3) and always have. I can't remember another time when this has not worked as it should. I am wondering if this is a problem created by a recent update.
Is anyone else experiencing this issue?
Any insight, or fix, would be greatly appreciated.

Answer 1

And only when it says Actual Size will the file be right. If it's set on small, medium or large files size will be reduced and will be turned into jpegs, no matter what the original version was.
So open up an email and attach a file and look at what it says. If it says anything but Actual size the file is already altered. So change it to actual size and save as draft, which you can then delete.
Then next time you attach an image it should say Actual Size because it defaults to whatever was last used. If it still changes then the file is probably too big, most emails have size limits and it's different for every service, so you have to check not only your own but the receiver of the email because if it's past their limit they will never get it.