Each one looked fine on its own, and then the set fell apart. It happens constantly when putting a deck together. The cause is not taste: specific things are out of step. Here they are, strongest effect first.
Match the drawing style first
Style matters more than anything else. Line art next to watercolour, flat next to 3D — mix those and the document itself looks assembled from scraps. However well organised the content is, this one point sets the impression.
Changing the order of your search makes it easier. Rather than finding subjects and trying to reconcile styles afterwards, settle the style first and look for subjects within it.
Then match the palette
Even in one style, warm and cool tones together fail to settle. If the document has a base colour, choose assets that lean towards it. If it does not, take the palette of the first image you picked as the reference.
Everything does not have to match exactly. As long as the base is shared, differing accent colours will not break it.
Composition can vary — the aspect ratio should not
Composition — subject centred, or pushed to one side — is better varied than matched; matching it makes the set monotonous. The aspect ratio is different. Landscape and square alternating down a page leaves the row heights uneven and the eye unsettled.
Deciding by use removes the question: landscape for slides, square for grids.
Match how people are drawn
When several assets contain figures, differences in proportion or in how much of the face is drawn put two different worlds in one document. If you are using people, choosing from within one style is the surest route.
Sometimes using fewer is faster
If matching is proving hard, consider using fewer images. One image with space around it reads better than three that do not agree. Illustrations are there to help understanding, not to fill space.
Finding matching assets on Visual Hub
Every asset here carries a style tag. Once you find one you like, following that tag lists only the assets in the same style — which is exactly the settle-the-style-first order described above.
Each asset page lists the style, orientation, composition and palette for every image on it, and the same subject often appears in several styles, so you can compare and take the one that suits the document.