Tasha Yar was in only the first season, so she could never be in the same scene as Dr. Pulaskie. Pulaskie and Crusher could never be in a scene together.
If Pulaskie, Crusher, and Yar not counted as main TNG characters, then the remaining six main TNG characters are:
1. Picard.
2. Riker.
3. Data.
4. Troi.
5. LaForge.
6. Worf
Seen in all seven seasons.
Plus:
7. Dr. Crusher seen in six seasons (one, & three to seven).
8. Wesley Crusher seen in about 3.25 seasons plus guest roles.
9. Guinan seen from time to time in the last six seasons.
And if some recurring characters are considered:
10. Miles O'Brien.
11. Keiko O'Brien.
12. Barclay.
13. Ro Lauren.
14. Alexander.
And of course Pulaskie and Yar could be and probably should be thought of as main characters.
15. Dr. Pulaskie.
16. Tasha Yar.
So the main TBG characters are definitely one to six, plus probably seven, eight and/or nine, plus probably fifteen and sxteen, making eleven reasonably main TNG characters, plus five semi demi main characters that some fans might possibly want to include.
So it is theoretically possible for:
A first season photo to show nine main characters (1-6, 7,8, & 16).
A season two photo to show nine main characters (1-6, 15, 8 & 9),
A season three and early season four photo to show nine main characters: (1-6, 7,8, & 9).
A later season four photo to show eight main characters (1-6, 7 & 9).
And for season five, six, and seven photos to show eight main characters (1-6, 7, & 9).
With numbers 10, 11, 12, 13, & 14 appearing in scattered episodes.