But you can't post images from your computer, can you?
No, they have to be hosted (aka stored) on a server that is controlled by a hosting service so they have a known place (aka link) on the internet that is accessible 24/7
If you tried to do that from you own computer the pics link would break (disconnect from the internet and be inaccessible) every time you shut off your computer, meaning no one would see it when your computer was off.
Ratsun does not provide this ability in house because it adds way too much to the cost of the already privately funded (aka out of pocket for one individual) forum that we all know and love. Some much larger forums do have this ability but the offset is banner ads and other BS we don't have to deal with here, and the mod and admin staff have no desire to deal with either.
This is an oversimplified explanation of the reasons but it seems that works the best for computer related issues on Ratsun.