To begin with, you'll need to log in to an account at Jigidi. (You don't need to register to solve jigsaw puzzles.) Once you've set up your account and signed in, a "Create" button will appear above the "Play" button. Click on the "Create" button to begin making your puzzle.
Press the "Select" button and browse to the image you want to upload. The number of pieces in your jigsaw puzzle depends on two factors: the size of the image (width & height) and its resolution (dots per inch/dpi).
Once you've uploaded our image, you'll see a screen similar to the illustration above with the image divided by horizontal and vertical lines. These lines indicate the approximate size of the individual jigsaw pieces.
To illustrate, the image above is a shot of the Louvre in Paris, which I'll make into a very simple 35-piece jigsaw puzzle. The image in my example is relatively small so there is a 45-piece limit for this image.
To the left of the image, drag the slider control up and down to expand and contract the indicator lines and preview the size of the final pieces. At this point, you might want to increase the size of the pieces (and thereby reduce the number of pieces) if the image has large areas with little variation in color and texture (such as a bright blue sky) which could be tedious to solve with too many pieces.
Once you're happy with the number of pieces and their cut, press the green "Test" button to see what your puzzle will look like disassembled.