Where are you posting it? That may determine how it will look.
For example, when you post an image to this forum the software may display a kind of down-sampled version of the image, rather than the higher resolution one you may have uploaded.
This might be caused by various things.
1) your browser adapted the zoom level to something other than 100%. Modern browsers are screen resolution "aware" and may zoom in a bit, resulting in somewhat blurry looking images. Check the browser zoom level.
2) when uploading an image to most (all?) social media sites, instagram, and many forums the image is processed and optimized once again. This will lead to image quality loss. To mitigate this somewhat, always upload a PNG file (you don't want to make things worse by reprocessing a jpg with compression artefacts), and the size you upload it at also impacts the overall compression quality. Upload an image that is identical or as close as possible to the final actual resolution used by twitter/facebook, etc., and scale down you images to the required size with a good downscale algorithm, such as Catmull-rom or Mitchell. Sharpen the result a bit before uploading. But that's really all you have control over.
3) the image is uploaded at a lower resolution and upscaled with CSS: obviously this will result in a softer image.
https://photography.tutsplus.com/tutorials/twitter-jpeg-compression-how-to-create-the-best-quality-image-for-your-feed--cms-23151
In a nutshell: don't expect the same quality as the original. You have relatively little control over how your images are mauled through, for example, Twitter's and Facebook's (and other sites) image processing servers. It is in their interest to keep the bandwidth as low as possible, and costs down. Which means a lot of lossy compression is used to force small-sized images, which in turn destroys the overall quality of your images.
The one way to prevent this from happening is to serve your images on your own server (or a dedicated image hosting service that does not reprocess your images), and use a forum img code that links to your externally served version. I found that hosting images on your own server works best: you retain full control.
Unfortunately, most social media sites will not allow for this, and want to take full control over your images: externally linking to an image is mostly not allowed or possible.
