What is the personal incredulity fallacy?

Concluding that because you personally can't understand or imagine how something could be true, it must be false.

The pattern: "I can't see how X could possibly work, therefore X isn't true (or didn't happen)." It fails because your inability to grasp something is a fact about you, not about the world — the limits of one person's imagination aren't the limits of reality.

"I can't imagine how something as complex as the eye could evolve gradually, so it must have been designed."

Not being able to picture the mechanism says nothing about whether it occurred; complex things can have explanations that simply aren't obvious to a given person at a given moment. The gap is in the understanding, not in the phenomenon.

Tip: "I don't understand how X" is the start of an inquiry, not the end of one — it's a fallacy the moment it becomes "…therefore not-X."