We have often talked about a size limit for pictures.
It could be way more useful if the dialog for uploading pictures would offer something like this, and if the size limit would not be enforced but offered in a convenient way.
For instance, I just edited two pictures into a new user's question. I could spend some minutes to download them onto my computer, fire up GIMP, scale them, and save them using an appropriate jpeg quality setting. But honestly, when I'm just editing someone else's question to help out, this would take too long and keep me from starting to embed the images in the first place.
Thus, my feature request: The box that opens whenever I upload a picture from my PC or from the web should offer two sliders: "Adjust Width" and "Adjust Quality/Compression". I figure this would require an image editor on the server anyway, so SE could also add "Brightness" and "Contrast" sliders, which would be handy to enhance badly lit photographs.