[2005] Copying Graphics (making graphics options make image tiny size)

Ok, currently my images range from 70-120 KB, I would like to get that down even further perhaps 40-50KB

these are my settings (and I can not change how much of the image is taken)

vb.net Code:

Dim gfx As Graphics = Graphics.FromImage(bitBuffer)
        gfx.CompositingQuality = Drawing2D.CompositingQuality.GammaCorrected
        gfx.InterpolationMode = Drawing2D.InterpolationMode.Default
        gfx.PixelOffsetMode = Drawing2D.PixelOffsetMode.None
        gfx.SmoothingMode = Drawing2D.SmoothingMode.None
        gfx.TextContrast = 0
        gfx.TextRenderingHint = Drawing.Text.TextRenderingHint.AntiAlias

the image im saving is a bitmap, and it is gotten from gfx.CopyFromScreen()
What settings can I set to those, to make the image size smaller, any idea's?
Thanks