You can see the code for the xmag utility, which probably operates by using XGetImage() and expands each pixel to be four pixels while writing into a second image, which is then converted into a pixmap for display. You can`t do much with pixmaps other than what the X protocol supports (creation, destruction, plane manipulation, fills), so you work with X Images; unfortunately, the Image library wasn`t ever finished, so you either work with raw data or use XGetPixel and XPutPixel (http//www.motifzone.com/resources/man/XPutPixel.html) .

For more complex operations such as rotations, see a graphics textbook such as "Fundamentals" by Foley Van Dam.