First step will be to open the highest quality available master that you have access to in Quicktime and advance the playhead to the frame that you would like to derive a still image file from.
In the navigation bar at the top of your screen, select Edit and navigate down to Copy (⌘C) and select this option.
Open the application Preview.
In the navigation bar at the top of your screen, select File and navigate down to New From Clipboard (⌘N) and select this option.
In the navigation bar at the top of your screen, select File and navigate down to Save (⌘S) and select this option.
First inputting your desired filename under the Save As field.
Next select the location for these files to be saved under the Where field.
Finally, you will select your desired Format and complete your export by clicking Save.
Please check the gallery of screenshots above for suggested settings to minimize compression for TIFF, PNG & JPEG assets.