nferyn;
I can't think of any online references that has pictures off the top of my head. If you go to a book store and look in the health/sports section, you should be able to find a few books on strength/weight training with pictures and descriptions. The New Encyclodedia Of Modern Bodybuilding (By Schwartzenegger & Dobbins) seems to always be in every bookstore in that section, it has pictures and straight forward descriptions of a variety of abdominal exercises.
Crunches - Are where you lie on the floor with your back on the ground and your ankles on a stool/bench (or against a wall), so that your knees are roughly 90 degrees from your waist. With your hands behind your head curl your head and trunk upwards towards your knees. Don't lift your entire back off the floor, but rather crunch the rib cage towards the pelvis. You can probably pick up an inexpensive exercise matt to make it more comfortable for you.
Roman Chairs - Sit on a bench or stool with your toes under a ledge for support. For the range of movement - lean back to about a 70 degree angle, but not so your torso is parallel to the floor; then raise and curl your body to an upright position. The rib cage and pelvis crunch together.
Knee tucks - Sit on the edge of a bench or a sturdy stool, and grip the front edge on either side of your hips with your hands for support. Hold your torso at a 45 degree angle, simultaneously; curl your torso toward your pelvis (rounding your back), and bring your knees to your head. The rib cage and pelvis crunch together. Then lower the knees and torso to the starting position. Takes a few times to co-ordinate and get the feel of what you are trying to accomplish.
With any of the above exercises, you want to use a slow movement, concentrating on keeping the abdominals under constant tension throughout the excercise. Don't wildly swing your torso/legs through the range of movement, and try to keep the range of movement within a set of exercises so that you have constant tension on your abdominals.
As far as the volume of exercise; you might want to try 3 sets of 20 reps to start out with, or go to failure (until you can't do anymore) for each set. Abdominals recover quickly, so you can do abdominal exercises almost daily without an issues. Don't be too aggressive when you start, no need to tear any muscles. If you ride at night, I would do the abdominal exercises in the morning to allow recovery. You don't want to do abdominal exercises immediately before cycling, as it will tire out your abdominals/lower-back, and perhaps lead to discomfort on your ride. Likewise doing the exercises immediately after your ride may over-stress your tired lower-back. If your experience muscular lower back pain, reduce the amount (volume and frequency) of abdominal exercises, and make sure to stretch your lower back (carefully).
Hope it helps,
-CG