Why does the size of my photos reduce to 0mb after editing in iPhoto?

Please bear with me here. I am a new Mac user (MacBook Pro, 2008, OSX 10.7.5), as well as not the most technologically competent person in the world, so I might need a little patience and additional questions to help me understand everything!

After a recent trip to Europe, I imported photos that I had taken with two different cameras into iPhoto ('09, version 8.1.2). All the photos that I took with my Canon Rebel t2i DSLR have not had the problem I'm about to describe, in the past or with photos from this trip. The problem has been only from pictures that I took and uploaded from my husband's Nikon Coolpix L610 point and shoot. Not sure if this will matter, but the mode I was shooting in was 16M (4608x3456) which appears to be the "best" quality for this camera.

As I was going through my photos and editing them (mostly minor edits, cropping, straightening, enhancing, altering the sharpness/exposure/definition/saturation), I noticed the size of my photos decreasing, as described in the information box in the lower left hand corner of the screen. With at least 30+ pictures of the ones I have tried editing (this isn't counting the hundreds more that I have yet to start editing), the file size dropped to 0MB, some after one edit as simple as straightening, others taking 2 or more minor edits to drop to 0. The picture is still there in the multiphoto view (in Photos section under Library), but if I try to click on the picture to view it larger, it just shows up as a black screen.

For example, one picture I have showed a size of 4608 x 3456 and 7.6 MB. When I did one edit (enhance photo), the 4608 x 3456 stayed the same, but the 7.6MB changed to 0KB. For others, it will decrease some with one edit, then drop down to 0 if I do additional edits. Most of my pictures are in the 4-10MB range in their original form (no edits). I'm assuming that I was shooting in jpeg mode, since that's what they uploaded as. (I've read on here of other people shooting in RAW mode and such, but I don't even think this camera has that capability). I read some other boards with similar questions, but not the exact same problem as I'm facing. I know that there is "jpeg compression" with jpeg files when editing.

So I guess my question is this: Is there anything I can do to stop this reduction down to 0? I will only be printing out a few of these pictures, and most in a 4x6 or 5x7 size. I'm more concerned with being able to post them to Facebook for my family, who lives far away. When the file size is 0KB, it won't upload because it doesn't even register as there being a picture and if I try it anyway, it gives me an error message, something along the lines of not supporting this format (or something similar, has been a couple days since I last tried).

I read something one of the similar boards that mentioned saving the edited photos as TIFF files, but it also mentioned that that takes up a lot more disk space, which I'm not sure if I want to (or have the space to) do. (Side question: If I did check the box in Preferences --->Advanced "Save edits as 16-bit TIFF files, would that do that to every picture in my library that I have edited?)

As I said, I'm new to this whole Mac stuff (been using it for just under a year, given to me by my brother-in-law after my computer died), so please use as much layman lingo as possible! And thank you in advance for anyone that can give me any insight and/or advice!