You have to bear in mind that for many people this is the first time they've ever used a forum or had anything to do with images on their PC. This makes it difficult to comply with the forum rules when they first attempt to post images.
I was used to working with images & their various formats long before I became interested in the Internet or simming so it was easy for me. I was also used to working with all kinds of graphics editors & knew how to resize images. I knew what 800 pixel width actually meant. Even so, I had a few problems when I first tried to upload an image & post it on this forum. Fortunately I have a knack for understanding instructions & soon figured it out. Other people seem to have all sorts of problems & it takes time for them to get the hang of it.
The main problem with trying to explain it on the forum is that if you use the correct code it doesn't display. It would be so easy if someone could copy/paste an example into their own topic but this this is not possible. In cases like this a picture is worth 1,000 words so a screenshot of the entry is far better.
I agree with Birdy that instructions on editing images & saving them in the appropriate format with IrfanView would be much better than Photoshop or PSP. Most newbies won't have complex graphics editors or know how to use them. I use IrfanView to process most of the screenhots & photos I post here.
I also think the instructions on the Image Upload page could be improved. The example path could easily confuse someone not used to doing these things. If the path is given
without the image like this:

The path can be copied from the webpage & pasted straight into the topic between those [img] brackets.
The image file name can then be added to the end of the path. This is how I always do it & I also copy/paste the image file name to the end of the path to avoid typos. This can be copied from the file properties of the image on your PC or from the box in the upload page when you upload the file.
A common cause of problems I've seen is that people don't realise the path is case-sensitive, including the image file name & file extension. Some graphics editors save .jpg as .JPG which is the most common error of the lot.