Pentium 4 CPUs were made and sold from 2000-2006, and don’t have any hardware graphics capabilities. You could add a graphics card to the computer, probably an older one with a PCI interface, because modern ones with a PCIe interface may not be compatible with your computer’s motherboard.
The free trial version of PixelCNC imposes a pixel-count limit on loaded images of 65,536 pixels, which means that you either must scale your images down to a width and height whose product is less than 65,536. This is to allow users to test drive all of PixelCNC’s features for free while limiting the amount of detail they can produce with it, particularly with larger projects like signs and CNC art.
For square images this means that 256x256 pixels is the size limit. For a 2:1 aspect image that would be 362x181, etc… You can downscale images pretty decently just using Paint, which is included with Windows 7. Just don’t overwrite your original images by saving the downscaled versions using the same file names!
The full version of PixelCNC imposes no hard limit on image sizes. However, it does perform some downscaling on images which are unnecessarily large for a given project size - to prevent redundant calculations from slowing things down. Otherwise it just uses the raw image itself for generating toolpaths. For signs my wife and make for Etsy using PixelCNC we use about 100 pixels per inch for project images. For a 24"x11" sign, for example, our images are 2400x1100 pixels.
EDIT: There should be some example images included with the trial version downloads that you can play with, which are already scaled down to satisfy the image-size constraints.