Graphic image quality issues

[carisma_sensei]

I know this isn't really related to the styles in Dreamwidth, but I don't know where to ask about this and, since most of us make graphics and use them in our layouts, I thought it would be a good place to ask for help. Please feel free to delete or ignore if irrelevant.

I noticed that when uploading graphics that are high quality (imagine, a Header that I want to use in a DW layout), the image gets reduced no matter what image hosting I use. In fact, when I preview it in my computer, it looks good, high quality and all that. But once I upload it to, say, imgur, it looks like they rescaled it a bit! Like they made it a bit bigger, I don't know why.
If I zoom out the browser, the image looks just like I saved it to my PC. But once I put the browser's zoom back to normal, it looks like they compressed the image and made it bigger too.

I don't know if this has happened to any of you, but being a designer and graphic maker, I'm a bit sad about this and I don't know what gives. Have you experienced this? I've had this with every image hosting service I tried. Am I going crazy? xD I don't do anything different and have been making graphics since 2006 so believe me I'm a bit confused about this.