What you need to know about photo files
High Quality JPG
Medium Quality JPG
Low quality JPG
Picture size, resolution, and file size are all separate things. Picture size is like its 8 x 10 inches in height and width. Resolution or DPI is dots per inch. How many pixels there are in one square inch of the photo. 300dpi is what you want for printing.
Note: if you scan an image at 300dpi but its a 4x5 picture the resolution is going to go down when you make it an 8x10. And 8x10 scanned at 300dpi would give you a perfect resolution for a printed image.
Note: you CANNOT improve the resolution of an image you already scanned by uping the dpi in an image editing program. If you scanned it at 72dpi and then up the res to 300dpi its still going to look like a 72dpi image.
File size depends on how you save it. If its a jpg, tiff, pdf, etc. JPG is the best format to save a file with the most compression and keep the best quality. JPGing an image reduces the size by taking away color information instead of resolution. Therefore if you jpg the image small enough when saving you can still have a 8x10 72dpi image below 100k if thats what you need it for an online photo contest. Best quality, minimum file size.
Re JPGing a JPG " See the pixels?
JPG qualities go like this 0-4 is low, 5-7 is medium, 8-12 "depending on your version of photoshop" is high. If you jpg a 8 x 10 72dpi color image for a photocontest and you save it as 8 - High quality it should be under 100k and you shouldn't be able to tell a difference on screen with the naked eye. Below a 5-medium your going to start to be able to see the difference. With the loss of color info you start to get a banding or pixely effect as shown in the photo above. This is especially visible if you rejpg a jpg so try to work from uncompressed file formats such as tiffs or eps if you plan on changing and resaving it.
Note: GIFs were made for the Internet also but don't bother saving your photos as a gif. They aren't as good a quality as jpgs when it comes to images which have gradients. They were made more for solid artwork than photographs.
Color
What's the difference between CMYK and RGB?
CMYK is Cyan Magenta, Yellow and Black
RGB is Red, Green, Blue
CMYK works on an ink basis for printing
RGB works on a light basis for computers
Your screen sees things in RGB and if you have a picture in
RGB that has very bright colors you may not be able to keep those colors when
printing.
The samples below show you what you get if you try to print the bright green
to the left.
Printer inks cannot achieve the color on the left without using spot color
which is a whole nother ballgame.
Why does the color look different from one computer to the next?
Not everyone's computer screen is set up the same. You will see color depending
on what you have your screen set at.
Why is my picture grey with lines through it when I put it on
the internet?
Or why does it not show up at all except for a tiny box with a red X?
Because you saved it as CMYK. Pictures on the net have to be RGB or Black
and white.