JPEG compression and DCT algorithm verification
I'll comment on readability as it could use some serious improvements.


Your indentation is very inconsistent.  Sometimes you indent two spaces, four spaces, eight spaces, or no spaces.  Choose a style and keep it consistent throughout the program.

Two common styles (using four spaces):

void func()
{
    if (isTrue)
    {
        // ...
    }
}




void func() {
    if (isTrue) {
        // ...
    }
}

Your use of whitespace is also inconsistent.  It'd be more readable to separate all operators and operands with one space and keep those consistent throughout the program.

Don't do this:

int number=1+2+3;


Do this:

int number = 1 + 2 + 3;

This comment:

//where n is the multi-decimal float


can just be placed above the function.  There's no need to put it in multiple places.
The comment next to applyDCT() should probably be placed on top of the function instead.  A comment that long should either be relocated or broken up into separate-lined comments.
These:

if (u == 0) *Cu = sqrt(1.0 / 8.0); else *Cu = sqrt(2.0 / 8.0); 
if (v == 0) *Cv = sqrt(1.0 / 8.0); else *Cv = sqrt(2.0 / 8.0);


are not very readable nor maintainable.  Prefer to apply curly braces, especially if you'll need to add to any of the conditions:

if (u == 0)
{
    *Cu = sqrt(1.0 / 8.0);
}
else
{
    *Cu = sqrt(2.0 / 8.0);
}

// the same applies to the second aforementioned line

Do not declare/initialize variables on the same line:

double Cu=0,Cv=0,cos_x=0,cos_y=0, DCT_value = 0;


This is also not very readable nor maintainable, especially if you'll ever need to add comments or modify individual variables.

Instead, have them on their own line:

double Cu = 0;
double Cv = 0;
double cos_x = 0;
double cos_y = 0;
double DCT_value = 0;


Moreover, in C++, you should try to use variables as close in scope as possible (next to where they will first be used).  This will reduce the need for long lists of variables like above.