- Poor scan. The scanner hardware and/or software may be doing a poor job. If the hardware, there's really no solution but to get a new scanner. If it's the software, you may be able to try another scanning application or tweak the current software. There may also be things you can do to improve the scan quality without doing either, such as clean the scanner's glass or print on a different type of paper.
- Overcompression when posting. Most posts on Internet sites are JPEGs, and JPEG is a lossy format. I don't know about APUG specifically, but some forum programs reduce the size of images and/or crank up the compression ratio, which degrades the quality of posted images. If these settings are hard-coded in the forum or image hosting site, there's not much you can do to overcome them, although pre-shrinking your images yourself and starting out with low compression (aka high quality) settings may do a bit to help. Alternatively, you could post the image on your own Web site and post a link rather than post directly to the forum.
You can determine in which class the problem belongs by comparing the image you upload to the image you see in your browser. If what you uploaded looks better than what your browser shows, then chances are #2 is happening; but if what you see before uploading looks similar to what your browser shows, then chances are #1 is happening. (One caveat: Browsers sometimes scale images themselves, and they usually do it badly. Be sure your browser isn't scaling an image. In Firefox, clicking a scaled image typically shows you the same image at full resolution.)