Numbers along the edges tell you which blocks of cells to fill. The picture emerges as you go.
Start with the longest runs. In a line of ten cells, a run of eight can only sit in three positions — and the six cells in the middle are filled in all three. That overlap is certain before you know anything else. Mark those, then look at what the neighbouring lines now force.
Most people only fill cells and never mark the empty ones. That is why they get stuck: the empty marks are what turn a vague line into a certain one.
Each nonogram in Puzelo is checked by a solver written separately from the generator, using a different method. A puzzle is only published if that independent solver finds exactly one solution — so you never need to guess, and the answer we print is never wrong.
More puzzle types: Sudoku · Binary Puzzle · Kakuro · Futoshiki