Photoshop :: Made Images Suddenly Look Very Bad Once Uploaded?

I have a blog and everything for it (header, banners, etc.) is made in Photoshop and up until yesterday it was all fine. But yesterday I wanted to try to put a scanned picture for my header - which on my computer looks white, but on the web it looked gray, so I decided to go back to what I had.

When I uploaded my old header it now looked blue/gray-ish, very pixelated and really strange. When I tilted my screen (laptop computer) I could see some weird edges around the shapes that I had on that header. I ulso tried uploading to Picasa web albums but the same thing happens.

I tried also making another image in Photoshop to test it out - it was white with big spots in green colour and when uploaded I could see the edges of the entire image even though it was white (it didn't blend with the rest of the background like it used to), and when I tilted my screen I saw pink edges around those big dots and it looked all fuzzy almost like it's moving (like on those very old TV's when you lose an image and it's snowy looking).