A concept that confuses people is the difference between the pixel size, resolution, and file size.
- this is the equivalent of inches or centimeters for images on the web. So if an image is 500 pixels by 500 pixels it will be a medium sized square on most monitors.
If it's 50x50 pixels - it will be a small thumbnail.
If it's 5000x5000 pixels it will be a humungous image and you will need to scroll up and down and sideways to see the image on most monitors. (Unless your monitor is the size of a huge TV)
resolution refers to how much information lies between the pixels. It can be measured in pixels, dots per (DPI) inch etc.
So for example, you can have an image that is 500x500 pixels, the resolution refers to how many 'dots' make up that image. The more dots - the sharper the image. (For printed images you need a lot of resolution to get a sharp image - for the web you do not, and images that ARE too large take a LONG time to load)
refers to the overall weight of the file based on pixel size and resolution. It is usually expressed in KB or K (kilobytes) or MB or meg (megabytes). It can easily be checked by right clicking on any image file and going down to properties.
Again for the purposes of uploading images on this board 100K is plenty - most of the images I upload are under 40k.
Hope that makes things a bit clearer