cron is actually quite easy. Do a 'crontab -e' to edit your crontab. It will fire up your default editor (possibly vi or nano) and probably an empty file. Here you can enter somethin like this:
0 * * * * wget -O /var/www/html/amps-0.7.5/amule.png 'http://dashaund.no-ip.info/amps-0.7.5/index.php?lang=en&sig_image'
That will execut the given command every full hour. The first five entrys tell cron when to execute this command. they mean:
field allowed values
----- --------------
minute 0-59
hour 0-23
day of month 1-31
month 0-12 (or names, see below)
day of week 0-7 (0 or 7 is Sun, or use names)
you can use * as a wildcard and comma separated values and some more fancy stuff, just enter 'man 5 crontab' in a console to take a closer look.