It is up to a group's owners and moderators to ensure that a group is worthwhile. The only groups worth joining are ones that have clear rules that are strictly enforced; if a group persistently has problems with users who dump pictures in a group irrespective of their relevance to the group and the moderators do nothing about it, leave the group. Unfortunately many groups are badly run. There is one group where I was briefly a moderator where the owners' response to moderators who enforced the rules was to remove the moderators. I subsequently left the group when the group, which was supposed to be geographically limited, began to resemble a campaign of world conquest. If there is a particular problem in a group, it may be worth complaining about it. I did so in one group that was plagued by a particular user who makes a habit of flooding every group to which he belongs with irrelevant photographs. The moderators set to work on his pictures and I was also made a moderator of the group, and the individual concerned, having continued to offend, was removed from the group, although he still continues to flood many other groups.
I think a group moderator's task would become much easier if it were possible for a group moderator to remove a picture from a group from the individual photo's page, just as the picture's owner can, instead of having to scroll through page after page of pictures searching for one offending picture.
It's not true that your pictures become invisible in a group once many others have posted pictures after yours; the facility to view a group's pictures on the map is very useful for this. Of course, it works better for groups that have a wide geographical spread; in groups that are geographically more limited more popular pictures will crowd the others out, but this is why it is probably a good idea for geographically limited groups to have limits on the number of pictures a user can post.