The problem I have now though is when I crop photos then upload them to my website part of the photo is missing, even though I resized the cropped pic back to its original size! I've always cropped using Paint and uploaded cropped pics to my website before with no problems, but for some reason it won't upload the full cropped pic now (theres like 3/4 of my picture sat there in the picture space on my website) - is this a website problem or is it me doing something wrong?
This will be harder to answer without more information about what kind of website you have. Wordpress, Moonfruit, Wix, other?
If you've cropped a picture (so it goes from being, say, 500 pixels high to being 400 pixels high) and then enlarged it to go back to its original size, you'll actually be stretching the pixels that are left post-crop. It's not a good idea to do this. It's easy to shrink images but you cannot make them bigger without creating some extra pixels. I may be misunderstanding you there, but just wanted to note that in case you were stretching your pictures out.
What part of the photo is missing when you upload the cropped pictures?
The height and width ratio of your images might be off now. So, if you picture was 500 x 500 pixels (height and width) but you crop half the face off the top, it'll now be 400 x 500 pixels. If you resize it upwards from the top-right-hand corner, it'll get taller and wider in proportion. So it'll now be 500 x 600 pixels. Your website might be expecting an image that's 500 x 500 so it'll shrink that extra width down to fit, and the gap at the top (those 100 pixels you cropped) will return.
You may be stretching the image upward only, though, to make them 500 x 500 again (sorry - just using those sizes for my example!), in which case this won't be the issue.
If you can PM me a link, I could have a proper rummage. Although not if it's a Flash-based site (Wix, and some Moonfruit, I think?); I can't help you there!