here is another important dichotomy in hand-drawn images: while some of them are rather symbolic, others are more decorative.  Symbolic images always refer to something real and external (a symbol is a symbol only when it symbolizes something), while the decorative ones may or may not depict anything recognizable, let alone related to the topic of the composition that they take part in.  Understanding the difference between these two classes, their uses and abuses, is a key to successfully using artwork in design.

Those decorative images that are based on a picture of some object or person are sometimes very detailed, realistic, and elaborative. For example, the contrast of such utmost realism with the absolutely surreal basic theme explains much of the appeal of Maurits Escher's works.  But more common (and more useful in design) is the sort of graphics whose decorative value is based on expressive texturization and various distortions, with not too much details and a generalized outline of objects (if they are present at all).

In symbolic imagery, on the other hand, generalization is king, and the outline is therefore much more important than texture, which tends to be very simple if present at all.  We can only perceive as a symbol a generalized depiction of an object, with minimum details, strong compositional balance, laconicism, and overall compactness (not to be confused with small size).  This sort of graphics is more "left brain," i.e. relying on the analytic capabilities of our perception, as compared to "right brain" decorative imagery.

Another relevant issue is serialization. If you take a series of illustrations by a single artist, all united by a common theme and common style, by using them in a single composition you are willingly or unwillingly shifting towards the symbolic end of the scale.  The eye of the viewer will try to find common features in the images, so as to support the idea of a series (especially if their homogeneity is expressed by formal means, such as positioning all images in the same place on a number of pages).  Whether you want this or not, this is essentially an act of generalization.

So, a group of images always leans to working as a symbol rather than a decoration, unless these images are spaced apart, for example by placing them at the beginning of each chapter in a book or on separate Web pages.  From another viewpoint, a series is only recognizable as such if it consists of at least three members.  This is why, if your online or paper publication is not really big, all decorative artwork should be restricted to a single image whose impact may be amplified by creatively reusing its fragments.  If, however, symbolism is what you're after, e.g. for a series of icons on navigation buttons, simple drawings united by a recognizable common style will work just fine.