Hi Luna,
A great trick I use when I'm drawing a blank is to look up pertinent keywords on a clipart or stock photo site, and see what sort of imagery they give me in response. The great thing about doing this is that if you find some imagery that you really like, then you can actually download it and use it! In fact, just looking at the keywords they list for a particular image is usually a great source of ideas for me.
My favorite stock photo site these days is
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You can download any photo they have, usually for a buck, and they have tens of thousands. Desaturating a photo and printing it out in black and white (or some other duotone) makes a great background for a page.
My favorite clipart sites are the ones published by Letters & Arts
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etc) because all of their art is available in vector EPS format (so it scales perfectly, prints at the highest resolution your printer supports, and has a transparent bkgnd). They work on a 1-price-for-unlimited-downloads basis, and usually have a cheap 1-week subscription where you can download as many images as you want for the duration.
So for example, for the "sis & dog" you could have dog paws walking across the page bkgnd, a desaturated photo or silhouetted outline of the dog in the bkgnd, a dog collar tag (with the dog's name would be really cool!), a toy the dog is known for (i.e. a Frisbee), a dog leash winding its way around the pages, etc.
Good luck, Noah