Need to join really big bitmaps together

Need to join really big bitmaps together

So I have an application that generates some bitmaps and then stitches
them together. This is fine in theory and in small scale but the problem
arises when I am stitching considerably large bitmaps together. First of
all I cannot generate a bitmap large enough for my bitmaps to fit in. In
my algorithm a large bit map is generated which is generated, such that it
can hold the others within it, then I paint the other onto it. I am doing
all of this in C# and using the System.Drawing.Bitmap class.
To be specific I noticed the problem in this setup:
Bitmap bmp = new Bitmap(resolution, resolution);
Where resolution = 25000.
So my question basically boils down to:
Is there an efficient way to join multiple bitmaps together without having
to max out memory?