there are basically 3 ways to reduce filesize;
1. photo quality
2. photo size
3. photo color depth
explaination;
1. this one depends on the compression format you're using. old skool formats like .bmp .png .tif etc. etc. all have a certain compression ratio. 1:10 for example. normally you cannot choose the compression ratio, however if you use .jpg or .jpeg you can choose the compression ratio anywhere from 1:100 when you save the file ;)
2. below are some example of standard sizes people usually save pictures as,
standard VGA is 320x240, same size as pictures from camera phones
SVGA is 640x480, minimum resolution for windows 95/98 full screen
XGA is 800x600, minimum resolution for windows xp full screen
normally use either VGA or SVGA resolution/size. BTW VGA/SVGA/XGA all are in landscape perpective if you picture is in portrait perpective flip the image 90 degrees then resize then flip it again, if not it would look weird :lol:
3. how many colors are on the image. btw the maximum is 16.7 millions of colors, coz the human eye can only see that much so anything extra is a waste of processing power and space. normally you can choose from;
a. monocrome, black & white, or 2 colors (1-bit)
b. 16 colors (4-bit)
c. 256 colors (8-bit)
d. 65536 colors (16-bit)
e. 16.7m (24-bit)
f. 16.7m colors (32-bit)
so why there is 24-bit and 32-bit color since its still 16.7m colors and the extra's are wasted?
"32-bit color" is for all practical perposes just 24-bit color that is aligned so that the color information for each pixel in the bitmap starts every four bytes vs. every three bytes.
The only reason this is done is that on the Intel 32-bit PCI/AGP/Memory data bus its more efficient to get at data that is "32-bit aligned". [with 24-bit data you often actually have to read the pixel's data twice--and then mask for the bits you are interested in]
The "fourth byte" is generally wasted.
so generally 32-bit is more efficient on computers ;)
so what the fuck all this means?
1. using JPEG/JPG compression should be about 75% for best compromise between compression and photo quality.
2. stick to VGA/SVGA resolution. you can also try reducing 25% from the picture if it is saved in an unusual size eg. wide frame class photo.
3. don't go down below 16-bit color for photographs. depending on the photograph. if it is a high quality professional SLR type photo going to 16-bit will spoil the image already making it look jaggy and non-uniform. for most digital camera/phone camera pictures going down to 8-bit color is still okay.
very useful ;)
test color depth
just additional info
web color pallette