1 there are two groups in this world: those who have a knack for assembling groups on the walls of their home or office, and then there is the group I am in.

2 while I am awaiting the wisdom for envisioning groups,given my line of work. I began dissecting, scientifically, that which is common to groups

3 theme.  a selection of images with something in common, like kids, or nature.  you are on your own here.

and a successful group is not made by a central theme alone,

4 because contrasting themes can also make a grouping by "differing" content, you might give that a little time in your imagination.

5 because differing themes sometimes make sense, there is another, logical way to think of a group.  it has things that stay the same, and some that differ, it is part of the same sort of thing.

6 if a grouping is all the same, it is boring, well usually.  a bunch of squares can make a good group, but usually it is boring. 

7 so lets establish that spacing should always be the same.  not that it works for me every time, however, some times constant spacing between wall art produces some odd shapes as a result.
 

9 note that the same setup works vertically & here there are two spacings, one for the in-between verticals, and then a central path down the central line.  differences at the outside periphery are desirable, even essential.  the group also has a small profile on right or left end growing in size toward the center.  Note that everywhere except that wider central line, a uniform space is a rule. 

10 I first saw this done (left) in an Pacfic Rim motif a couple dozen years ago, and I was amazed how well it worked.  some considerations must be kept in mind.  this must be the only thing of its kind; a small space of wall is ideal, it has to have its own space.

11 groupings must have an uniform spacing, and they should have very dissimilar dimensions on two axis. 

12 where possible try to have one or maybe two large images, and the others make the rest of the group work if they are somewhat smaller.