There are a few things you need to consider when choosing your images:
1) Consider the character's personality and emotions
- for example if you want to make a wall with Hitsugaya, don't pick a background with flowers and petals, it doesn't fit with his personality
- if the character is crying and is obviously sad, not a happy cry, don't choose a joyful background filled with happiness and rainbows
- in order to emphasize the emotions, the background must mirror the character/s
* this scan goes very well with this scan (the colors fit and candy are Ciel's favorites, fit him as character)
* but it would look horrible with this one (wrong era and colors too dark)
2) Don't use opposing colors
- the pictures you combine must have colors in common, or colors that go well together; a serious wallpaper shouldn't have purple, green and yellow as the main scheme
- even the funky walls filled with a big number of crazy hues have colors in common which is why the flow well together (example)
- don't use colors uncharacteristic to the character, unless it's a funky wall
* this scan goes great with this one
* but not so much with this (too orange)
3) The images mustn't have entirely different execution styles
- if you want to use a vector, don't choose a background that looks entirely painted, there's no blending, it excludes the character
4) Match the outfit
- don't pick a Victorian castle if your character wears a modern punk outfit
- you could create a contrast if the character looks amazed, as if he stepped in another world, then yes, two different era images could go well together
Tricks to match the colors:
If you really want to use images that don't have friendly colors, the best way to make them work is to play with the saturation of the brightest one. Lower its Saturation a bit, after that sometimes it's needed to increase the Contrast a bit.
After you're done color playing, I strongly suggest Photo Filter (Photoshop has it, I'm not sure about others). Pick the dominant color in your wall and set it as Filter at what intensity you want. It should equal the colors a bit, enough to make then go better.
If you don't have Photoshop simply fill a layer with the dominant color and play with blending modes until you reach the wanted result.