Mom could be nervous or afraid of dad. We don't know. And the OP says "we" have tried reasoning with dad, so that implies that it could be one or both of them. It's not unreasonable for the OP to speak for or with mom if that's her choice. Granted, dad doesn't have to legally abide by stepdad's wishes, but, it should not be about anyone's wishes - it should be about the child. And, unless the child was forced to play football against his will, the people that appear to have a greater focus on the child are mom and stepdad.
If dad were focused on the child, he shouldn't have a problem with what appears to have been proposed. If he's refusing to allow the child to have pictures with the team simply so he can get 15 more minutes to stare at his kid, that's darn selfish if you ask me.
I'd still like to know if dad is blocking practice every week, or if it is only this one picture day that is the issue. I have known children who grew up resenting a parent that refused to allow them to play sports or engage in other extra curricular activities because they had a pissing match with another parent. If this child will be denied sporting or other activities (gymnastics, theater, music, etc.) because a parent prefers to have exclusive time for themselves, then that parent runs the risk of alienating that child.