Depends how you export your picture. If you export it as a Compuserve Bitmap (.gif), it will be palletized, or converted to a set of predetermined colors. The image is smaller that way, since ever color has a set value instead of being generated numerically. Like the computer says "this is red" instead of "this is color number 6852133." So you will need to tell the program how you want the colors arranged. As you export the GIF a dialogue box will pop up asking you a few questions. The important thing here is "dithering" witch is like a pattern that the program lays the colored dots on to make the image. If "dithering" is set to ordered the color is layed on regular criss-crossing lines that make that dotted effect you were referring to. You can do one of two things: either turn "dithering" to none or choose the setting from the list that visually makes your image look the best. There really is no perfect setting, the texture of your image will determine which works the best.
For a JPEG (.jpg) export, you want to turn down the compression and smoothing settings, and possibly adjust the "sub-format" to 4:4:4 or higher.