Size or symmetrical balance: it is a good idea to avoid “heaviness” on a page. For example, putting a large picture in one corner and no other pictures and little embellishment. If you have a large item you want to offset, it is a good idea to put 2 smaller pictures in the opposite corner or side to help balance it; you can also use cutouts, decorations and other embellishments in a way that the one side does not appear “heavy”. This can also work with 2 pictures and 3 smaller pictures/embellishments; you can also create a type of interesting balance with the same number of pictures in opposite corners.
Color can be an interesting thing to balance: you can use it to create a mood on your page, balance out less desirable elements or colors in a picture or enhance a feature or color of a picture. For example, if you have an orange hat in a picture that’s just *too* bright and pulls to much attention, you could half-mat it on the corner nearest the hat with red and that will help “fade” it, or make it less obvious. However, that same red mat would make the greens in a forest shot really stand out. Usually the colors on a color wheel nearby the one in question will help blend it in, colors across from it on the wheel will enhance it with contrast.
You can also use “rhythm” to balance your page. Having straight lines, even spaces, balanced shapes (square, circle, triangle) and small patterns can give an “organized” or “controlled” rhythm. Having diagonal lines, unequal shapes (pentagon, parallelogram, polygon), uneven spaces and unequal sizes or patterns can give a “dynamic” or “active” feel.
You can also use your embellishments (cutouts, stickers, borders, gems, etc.) to a balancing effect with your picture: a simple, plain picture such as a portrait might be overwhelmed with lots of embellishment, where a busier picture might benefit from more details around it.
If you have a hard time balancing your page, lay your items in place and walk away for a while, so you can see it later with a fresher perspective. Or, have a friend or family member who has a good eye for these things check it for you before you make your page permanent.