My maximum screen resolution is 1900 x 1200 pixels, so that's what I send to Postimage. Anything in excess of screen resolution gets down sized to display anyway. Seems everything gets resized though as they go from server to server. If I go back to Postimage and open an image it gets displayed at 1165x728. If I download that same image, it comes back and is saved at the original resolution of 1900X1200.
The above is a 1900X1200 image saved on the Postimage site.
When displayed on my Campfire link and this Lenovo screen it's up at 1280X800. The original coming out of Cookie's camera is 4200+ something pixels wide and a 28 MB image.
Anything exceeding most screen resolutions is simply consuming extra file space and extending download time.
Tikkanut's sheep immediately above this post displays on my machine at 1425x950. If I right click the image, go to its properties, copy its address, and go to the imgur site, it displays at 1581x 1054. It too is saved at a much larger resolution, and if I save it from the imgur site it comes to my machine at 5184x3456. Some imgur images posted here, seem to take forever, to come up, likely because they were saved as huge files. Such seems to be the case with Tikkanut's sheep.
The highest resolution monitor I can surf up is a Dell at 7680 x 4320 with no info on pricing or the graphics card needed to power such. Likely something most of us will not acquire unless we're into professional grade photography.
Your results might vary depending on your screen's abilities. For most internet stuff, huge high resolution images are a waste. If one is routing something out to be printed, however, maximize resolution and file detail in a *.tif format.
To put it briefly, I'd suggest taking images down to your screen resolution if ones purpose is simply web display. Never, however, downsize ones original images for archival purposes.