How can I export a still image in Quicktime Pro 7.7 so that it is readable in the Photoshop CS6?
Basically you can't. As you noted above, the QT 7 Pro option only exports a PCT file (the legacy system image file format) using your choice of compression.
You could, however, try any of a number of other options:
1) You can drag the current frame in your QT 7 Pro player to the desktop which should create a single-frame "Movie Clipping" file in the same format as your video video file. This file may or may not open directly in your updated version Photoshop as it does in my older version.
2) Another possibility would be to use the Command-C keyboard shortcut to copy the current image in the QT 7 Pro player to the clipboard and then open Photoshop and create "New" file in Photoshop using the clipboard image dimensions. When the new empty file window opens, you can use the Command-V shortcut to paste the clipboard image to the window. On my system this creates a default PSD image. The major disadvantage of this approach is that if the original file was encoded anamorphically, then the image is created using the encode dimensions and you have to manually adjust the aspect ratio. (I.e., as opposed to using the work flow described above which recognizes anamorphic files and gives you the option of opening the file automatically in the correct apect.)
2) If the "Movie Clipping" will not open in your version of Photoshop, then it will open in the free MPEG Streamclip app if you have it installed. This app can then export the video frame as a JPEG, TIFF, or PNG image.
3) Another option would be to open the AVI video directly in the MPEG Streamclip, locate the frame you want and export the frame directly from the video file to a JPEG, TIFF, or PNG file as described above.
4) Probably the most used work flow for compatible video is to open the file in the QT X Player, locate the frame you want, use the Command-C shortcut to copy the image to the clipboard as a PNG image which can the be opened from the clipboard in Preview and exported to a desired image file format. Of course, AVI files are frequently "iffy" with QT X depending on the specific video compression format used to create them.
5) You can also use the Finder image capture option to capture the entire QT 7 Player image (or a portion thereof) as a PNG image (with or without a drop shadow) and then open it Photoshop where you can crop the image before performing whatever you want to do in this app.
The are probably other options but this should be enough for you play with at this time.