Yes, if the change occurs after the file is saved to disk, and the file is a lossy compressed format such as .JPG, the poor definition of text is at least partly due to the artifacts from compression. .BMP files should be OK, as they normally default to an uncompressed format, and also other uncompressed or lossless compression formats such as .TIF, .PNG and in limited circumstances, .GIF.
Jove, if you are working with an image, it should be saved in a lossless file format and if necessary converted to a lossy format like .JPG only as a final step when completed. It's still inevitable then that there will be some degradation of text quality, but it can be minimized by selecting the least amount of compression that will deliver an acceptable filesize.
The resolution of the image will also affect how "true" the representation of text can be, as it controls how many pixels form the characters. So if you are creating a bitmap image to
print, make the image pixel dimensions as high a resolution as feasible. In contrast, if the image is for a screen viewing application, work at the required resolution from the start. If you need a 640x480 image for a webpage, don't start with a much larger canvas, then re-size it when completed, as the re-sampling will further degrade the edges of text. With a bitmap, there are only so many pixels to "draw" a given shape.
Simple work-arounds like these can help to get the best results from simple bitmap graphics programs like Paint. As shame228 comments, if you need to do more than Paint can manage, you'll need to move to more sophisticated software.
http://www.wfu.edu/~matthews/misc/graphics...ts/formats.html