The Commodore 64 has two hardware modes, high resolution and low resolution (also called multi color mode). In addition to those there some software assisted resolution (no image format for those are yet supported by abydos). Most image formats used on the C64 are for low resolution, 160x200 pixels. Since the images are meant for TV:s with a 4:3 screen ration, abydos assumes the pixel aspect ratio of 5:3. This is however not entirely accurate. First of all the image doesn't occupy the whole screen, there is a border around the edges. The actual size of this border may vary slightly depending on the TV and it's settings. But more importantly it pends on whether NTSC or PAL is used. NTSC gives a pixel aspect ratio of 3:2 and PAL 234:125. So 5:3 could be considered a reasonable compromize. For more information on this please read about accurately reproducing the Video Output of a Commodore C64.
The palette used on the C64 is often called VIC-II after the chip used to generate it. It uses YPbPr encoding of the colors so there are no definite RGB representations. And coming up with a suitable palette is no easy task. Especially since the actual appearance of colors depend very much on the contrast setting of the TV screen. As a reasonable compromise abydos uses the colodore palette with 50% contrast.
The colocore palette.
This is what a rendition looks like with the current version of abydos.