You're missing a couple of key elements here (and I read your post in the composition thread, too).
The file size (in MB) depends on a number of things, but we can reduce those to two for the purpose of your question:
1) The actual size (in pixels) of the original photograph itself (if it's a 6mb shot coming out of a Canon, I suspect that they're ~5184 x 3456 pixels) - think about that as the x,y axis (like a 3x5 index card - that's 15 square inches). That's the first factor.
2) The depth of information in the picture. For example, a color photograph has more information than a black and white - more colors = more information - and therefore is going to result in a larger file size. Think of that as the z-axis...or the "thickness" of the photograph.
So what you're trying to do is get your photos from 6mb to 80kb (which is pretty tiny).
The first question thing to do is figure out how big they are when they're displayed. Even if we reduced their x,y dimensions by a factor of ten - to 518x346 and they were only going to be displayed at 259x158, they'd be four times larger than they needed to be...that is to say that they'd have four times the information (file size) than what would be displayed. So, you need to find out, in pixels, how large the pictures are when they're displayed on the site. So get those dimensions.
After you have those, the process will be:
1) Crop the photo to contain whatever you want. Don't worry about the size (except in rough x,y proportion - if it's going to be displayed at 300x200 pixels, make the width of the crop roughly 3x the height). Just get it looking like you want. We'll fix the file size in a minute.
2) When you save the crop, most photo editors will allow you to save it at the dimensions you choose (and will do the math to keep it proportional). For example, you can tell it to save the crop at 300 pixels wide.
3) Check the file size of what you've saved. If it is more than 80kb, go to step 4.
4) You're likely saving the pictures as a .jpg. That's the file type. And if a .jpg's file size is larger than you want it, you can decrease the information that it contains by using your photo editor. For example, in microsoft's photo editor you would go to Picture ---> Compress Pictures and select one of the radio buttons.
Look at the screen shot below. The picture is some random thing from ATP (apologies, Vickie) with an original file size of 281kb (and is 1000x667 pixels in its original form). By telling it to compress for "Web Pages", it knocks the file size down to 44.1kb (well within your limits) but also reduces it to 488x299 (cutting it roughly in half).
Well, shoot. That wasn't exactly what I had in mind. It looks like when microsoft "compresses" pictures, it doesn't do anything with the depth of information on the z-axis, but rather shrinks it while keeping the quality of the .jpg.
Most other photo editors will allow you to specify the "quality" of the .jpg which, really, is the depth of information...the z-axis above.
If this doesn't make sense...or you're unable to get them small enough after following these instructions, post here and let me know what photo editor you're using.
But shooting from afar or shooting to crop or even shooting at a lower resolution isn't the right answer. Dealing with the size of the file proper is.