The hard part might be creating the graphics that you want to use on your PCB silkscreen. The bulk of my time trying to figure this out was trying to find the right combination of sizes and colors of the bitmap for my graphics. I searched around forums
and websites trying to find some right answers. Hopefully it will be easier for you.
Most PCBs are small. The graphics will look much bigger on your computer screen than they will look on your PCB. Most PCB silkscreen is white. On your computer screen you want the artwork color to be black.
Create a large size bitmap (BMP) with a size of 2000x2000 at least.
Add your artwork by either pasting it in or creating it new on the canvas. Make sure the color mode is set to grayscale or monotone. If you have some existing artwork, just overlay the artwork with black. That's what I did with our logo. In EAGLE we will be only importing one color.
To gauge the appropriate size, use 175 pt font on the artwork. When this gets added to the actual silkscreen the size will be around a 1/4" high. You can experiment with the size depending on how much room your logo or artwork needs to take up vs. the size allotment on your PCB. From my experience, every 175 pt high font is about 1/4" on the printed circuit board.
Remember if it looks big on your computer screen, then you are probably doing things right so far. On my attached screenshot, I am zoomed out to 33% and it still looks big.
Save the final design as a bitmap (BMP) at 8-bits at the most.