I kind of like the way World of Warcraft names their screenshots.
You have whatever name you wish, followed by the MMDDYY_HHMMSS (although I prefer YYMMDD_HHMMSS myself, the files will then be listed in the right order in a directory). This gets rid of any limits on how many you can do in a day because they will never be over written and it makes it easy to know when you took the screenshot at a glance.
You could also simply do a "screenshot%d" or "screenshot%03d" and add on a number you increment for each shot.
The function prefix could also be "a5_screenshot" if you're looking for something that is easily recognizable other than "al_".
Edit: I would also prefer it to be automated so that nothing needs to be passed to the function, just have it get the executable filename, crop the ".exe" off of it, then save the screenshot into the same folder that the executable resides in, perhaps creating a "screenshots" folder if one doesn't exist and saving all screenshots in there.