float sourceWidth=192.0f;
float sourceHeight=128.0f;

float scaledWidth=sourceWidth*1.5f;
float scaledHeight=sourceheight*1.5f;

// First we need to find a ratio between the source image and the scaled image
// We already know it's 1.5

float ratioX=scaledWidth/sourceWidth;
float ratioY=scaledHeight/sourceHeight;

// Then we need to loop through each scaled image pixel

for(int i=0;i<scaledWidth;i++)
{
    for(int j=0;j<scaledHeight;j++)
    {
        // Time to get the real number position on the source image

        float sourceX=(float)i/ratioX;
        float sourceY=(float)j/ratioY;

        // Then we find the four pixels around our position

        int minX=floor(sourceX);
        int minY=floor(sourceY);
        int maxX=minX+1;
        int maxY=minY+1;

        // We need to check if the bottom-right pixel is out of the source image

        if(maxX>=sourceWidth)
            maxX--;

        if(maxY>=sourceHeight)
            maxY--;

        // Now we get the distance between the top-left source pixel and
        // The real number position

        float dX=sourceX-(float)minX;
        float dY=sourceY-(float)minY;

        // And for each color component (R,G,B) we apply the formula
        // You can find it in the second link I posted

        // I assume I have both GetPixelComp and SetPixelComp functions
        // And both images represented as source and scaled

        for(char k=0;k<3;k++)
        {
            unsigned char component=(unsigned char)
            (GetPixelComp(source,minX,minY,k)*(1-dX)*(1-dY)+ // Top-Left
            GetPixelComp(source,maxX,minY,k)*dX*(1-dY)+ // Top-Right
            GetPixelComp(source,minX,maxY,k)*(1-dX)*dY+ // Bottom-Left
            GetPixelComp(source,maxX,maxY,k)*dX*dY); // Bottom-Right

            SetPixelComp(scaled,i,j,k,component);
        }
    }
}