What format is the image in memory?
If you have an actual Bitmap object, just assign it to the PictureBox, as suggested by dtb:
pictureBox.Image = bitmap;
If you have the image as a series of bytes held in a stream, you'll need to load the image from the stream:
var image = Image.FromStream(stream);
pictureBox.Image = image;
If you instead have a windows GDI handle to the bitmap, use
var image = Image.FromHbitmap(handle);
pictureBox.Image = image;
Essentially, it's hard to answer your question with more than suggestions when you haven't told us what format the Bitmap you have is held in.
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