I had a problem with Photoshoshop CS3 for about a week and it was driving me nuts. For some reason when I tried to save the files it wouldn’t give me an option to save an .png. I double checked that the image was in RGB Mode (Image>>Mode>>GRB Color),since png option is not available in CMYK and kept it in 8bt resolution (Image>Mode>8bits).
I could save everything as “.psd”, “.jpg” or another file type except “.png”. It just did not appear in the drop-down menu when I select “File – Save As”.
Well, after a lot of research, I came across “Adobe Security bulletin”.(http://www.adobe.com/uk/support/security/bulletins/apsb07-13.html) Click here.
I Manually updated the security patch, and there it was, “Png.8bi” plug-in. I have no clue why the plug in got deleted but apparently a lot of people are having the same problem.
In Windows Explorer, navigate to the following location(s):
For CS3: C:\Program Files\Adobe\Adobe Photoshop CS3\Plug-Ins\File Formats;
In the Finder, navigate to the following location(s):
For CS3: Applications//Adobe Photoshop CS3/Plug-Ins/File Formats
I hope this helps. Good Luck!