There are two very different cases, and it's critical to recognize the difference.I notice that uploaded images sometimes look less sharp on the forum than they looked on my screen when I uploaded them. Why is that?
If the image to be uploaded is less than or equal to 1024 pixels in width and height, and the file length is less than or equal to 300 KB, then what is served back will be identical to what was uploaded. If there is a problem with sharpness in this case, it is between your browser and your offline image viewer -- nothing to do with the forum software.
But if the image to be uploaded is greater than 1024 pixels in either X or Y, then it will be resized by the forum software. That resizing often introduces some blurring. The solution to this problem is to do the resizing yourself, so that you can control the interpolation method and apply additional sharpening if desired.
Summary: Do not let the forum software resize your images for you!
One last case is sometimes very confusing. If the image to be uploaded is less than or equal to 1024 pixels in width and height, but the file length is greater than 300 KB, then the image will not be resized by the forum software, even though intuition says it ought to do that. Instead, the upload will simply be rejected. Clearly this has nothing to do with softness. I am mentioning it here only for completeness, because it's related to the width/height/filesize numbers.